build: Replace `make config` with a `./configure` script

This lets us do more traditional out-of-tree builds like

    $ ../path/to/bfs/configure
    $ make

The .mk files are moved from ./config to ./build, mostly so that
./configure will auto-complete easily.

63 files changed, 445 insertions, 387 deletions

diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 3890bb0..57f59f4 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -107,7 +107,7 @@ jobs:
                 gmake \
                 oniguruma
             jobs=$(sysctl -n hw.ncpu)
-            gmake -j$jobs config
+            MAKE=gmake ./configure
             gmake -j$jobs check TEST_FLAGS="--sudo --verbose=skipped"
 
   netbsd:
@@ -133,7 +133,7 @@ jobs:
                 pkgconf \
                 tcl-expect
             jobs=$(sysctl -n hw.ncpu)
-            make -j$jobs config
+            ./configure
             make -j$jobs check TEST_FLAGS="--sudo --verbose=skipped"
 
   dragonflybsd:
@@ -165,7 +165,7 @@ jobs:
           run: |
             chown -R action:action .
             jobs=$(sysctl -n hw.ncpu)
-            sudo -u action make config
+            sudo -u action ./configure
             sudo -u action make -j$jobs check TEST_FLAGS="--sudo --verbose=skipped"
 
   omnios:
@@ -198,5 +198,5 @@ jobs:
             PATH="/usr/xpg4/bin:$PATH"
             chown -R action:staff .
             jobs=$(getconf NPROCESSORS_ONLN)
-            sudo -u action gmake config
+            sudo -u action MAKE=gmake ./configure
             sudo -u action gmake -j$jobs check TEST_FLAGS="--sudo --verbose=skipped"
diff --git a/.github/workflows/codecov.yml b/.github/workflows/codecov.yml
index 2245f40..bb68927 100644
--- a/.github/workflows/codecov.yml
+++ b/.github/workflows/codecov.yml
@@ -25,7 +25,7 @@ jobs:
 
       - name: Generate coverage
         run: |
-          make config GCOV=y
+          ./configure GCOV=y
           make -j$(nproc) check TEST_FLAGS="--sudo"
           gcov -abcfpu obj/*/*.o
 
diff --git a/.github/workflows/codeql.yml b/.github/workflows/codeql.yml
index c21fda5..a0b8fe3 100644
--- a/.github/workflows/codeql.yml
+++ b/.github/workflows/codeql.yml
@@ -41,7 +41,7 @@ jobs:
 
       - name: Configure
         run: |
-          make -j$(nproc) config
+          ./configure
 
       - name: Initialize CodeQL
         uses: github/codeql-action/init@v3
diff --git a/Makefile b/Makefile
index 981c322..2fb35fd 100644
--- a/Makefile
+++ b/Makefile
@@ -1,56 +1,50 @@
 # Copyright © Tavian Barnes <tavianator@tavianator.com>
 # SPDX-License-Identifier: 0BSD
 
-# This Makefile implements the configuration and build steps for bfs.  It is
-# portable to both GNU make and most BSD make implementations.  To build bfs,
-# run
+# To build bfs, run
 #
-#     $ make config
+#     $ ./configure
 #     $ make
 
 # Utilities and GNU/BSD portability
-include config/prelude.mk
+include build/prelude.mk
 
 # The default build target
 default: bfs
 .PHONY: default
 
 # Include the generated build config, if it exists
--include ${CONFIG}
+-include gen/config.mk
 
-## Configuration phase (`make config`)
-
-# The configuration goal itself
-config::
-	@+${MAKE} -sf config/config.mk
+## Configuration phase (`./configure`)
 
 # bfs used to have flag-like targets (`make release`, `make asan ubsan`, etc.).
 # Direct users to the new configuration system.
 asan lsan msan tsan ubsan gcov lint release::
-	@printf 'error: `%s %s` is no longer supported. ' "${MAKE}" $@ >&2
-	@printf 'Use `%s config %s=y` instead.\n' "${MAKE}" $$(echo $@ | tr 'a-z' 'A-Z') >&2
+	@printf 'error: `%s %s` is no longer supported. Use `./configure %s=y` instead.\n' \
+	    "${MAKE}" $@ $$(echo $@ | tr 'a-z' 'A-Z') >&2
 	@false
 
-# Print an error if `make` is run before `make config`
-${CONFIG}::
+# Print an error if `make` is run before `./configure`
+gen/config.mk::
 	@if ! [ -e $@ ]; then \
-	    printf 'error: You must run `%s config` before `%s`.\n' "${MAKE}" "${MAKE}" >&2; \
+	    printf 'error: You must run `./configure` before `%s`.\n' "${MAKE}" >&2; \
 	    false; \
 	fi
 
 ## Build phase (`make`)
 
 # The main binary
-bfs: ${BIN}/bfs
+bfs: bin/bfs
 .PHONY: bfs
 
 # All binaries
 BINS := \
-    ${BIN}/bfs \
-    ${BIN}/tests/mksock \
-    ${BIN}/tests/units \
-    ${BIN}/tests/xspawnee \
-    ${BIN}/tests/xtouch
+    bin/bfs \
+    bin/tests/mksock \
+    bin/tests/units \
+    bin/tests/xspawnee \
+    bin/tests/xtouch
 
 all: ${BINS}
 .PHONY: all
@@ -60,52 +54,50 @@ ALL_CFLAGS = ${CPPFLAGS} ${CFLAGS} ${DEPFLAGS}
 ALL_LDFLAGS = ${CFLAGS} ${LDFLAGS}
 
 # The main binary
-${BIN}/bfs: ${LIBBFS} ${OBJ}/src/main.o
+bin/bfs: ${LIBBFS} obj/src/main.o
 
 ${BINS}:
 	@${MKDIR} ${@D}
-	+${MSG} "[ LD ] ${TGT}" ${CC} ${ALL_LDFLAGS} ${.ALLSRC} ${LDLIBS} -o $@
+	+${MSG} "[ LD ] $@" ${CC} ${ALL_LDFLAGS} ${.ALLSRC} ${LDLIBS} -o $@
 	${POSTLINK}
 
 # Get the .c file for a .o file
-_CSRC = ${@:${OBJ}/%.o=%.c}
-CSRC = ${_CSRC:gen/%=${GEN}/%}
+CSRC = ${@:obj/%.o=%.c}
 
-# Depend on ${CONFIG} to make sure `make config` runs first, and to rebuild when
-# the configuration changes
-${OBJS}: ${CONFIG}
+# Rebuild when the configuration changes
+${OBJS}: gen/config.mk
 	@${MKDIR} ${@D}
-	${MSG} "[ CC ] ${_CSRC}" ${CC} ${ALL_CFLAGS} -c ${CSRC} -o $@
+	${MSG} "[ CC ] ${CSRC}" ${CC} ${ALL_CFLAGS} -c ${CSRC} -o $@
 
 # Save the version number to this file, but only update version.c if it changes
-${GEN}/version.c.new::
+gen/version.c.new::
 	@${MKDIR} ${@D}
 	@printf 'const char bfs_version[] = "' >$@
 	@if [ "$$VERSION" ]; then \
 	    printf '%s' "$$VERSION"; \
-	elif test -d .git && command -v git >/dev/null 2>&1; then \
-	    git describe --always --dirty; \
+	elif test -e src/../.git && command -v git >/dev/null 2>&1; then \
+	    git -C src/.. describe --always --dirty; \
 	else \
 	    echo "3.1.3"; \
 	fi | tr -d '\n' >>$@
 	@printf '";\n' >>$@
 
-${GEN}/version.c: ${GEN}/version.c.new
+gen/version.c: gen/version.c.new
 	@test -e $@ && cmp -s $@ ${.ALLSRC} && rm ${.ALLSRC} || mv ${.ALLSRC} $@
 
-${OBJ}/gen/version.o: ${GEN}/version.c
+obj/gen/version.o: gen/version.c
 
 ## Test phase (`make check`)
 
 # Unit test binaries
 UTEST_BINS := \
-    ${BIN}/tests/units \
-    ${BIN}/tests/xspawnee
+    bin/tests/units \
+    bin/tests/xspawnee
 
 # Integration test binaries
 ITEST_BINS := \
-    ${BIN}/tests/mksock \
-    ${BIN}/tests/xtouch
+    bin/tests/mksock \
+    bin/tests/xtouch
 
 # Build (but don't run) test binaries
 tests: ${UTEST_BINS} ${ITEST_BINS}
@@ -117,45 +109,45 @@ check: unit-tests integration-tests
 
 # Run the unit tests
 unit-tests: ${UTEST_BINS}
-	${MSG} "[TEST] tests/units" ${BIN}/tests/units
+	${MSG} "[TEST] tests/units" bin/tests/units
 .PHONY: unit-tests
 
-${BIN}/tests/units: \
+bin/tests/units: \
     ${UNIT_OBJS} \
     ${LIBBFS}
 
-${BIN}/tests/xspawnee: \
-    ${OBJ}/tests/xspawnee.o
+bin/tests/xspawnee: \
+    obj/tests/xspawnee.o
 
 # The different flag combinations we check
 INTEGRATIONS := default dfs ids eds j1 j2 j3 s
 INTEGRATION_TESTS := ${INTEGRATIONS:%=check-%}
 
 # Check just `bfs`
-check-default: ${BIN}/bfs ${ITEST_BINS}
+check-default: bin/bfs ${ITEST_BINS}
 	+${MSG} "[TEST] bfs" \
-	    ./tests/tests.sh --make="${MAKE}" --bfs="${BIN}/bfs" ${TEST_FLAGS}
+	    ./tests/tests.sh --make="${MAKE}" --bfs="bin/bfs" ${TEST_FLAGS}
 
 # Check the different search strategies
-check-dfs check-ids check-eds: ${BIN}/bfs ${ITEST_BINS}
+check-dfs check-ids check-eds: bin/bfs ${ITEST_BINS}
 	+${MSG} "[TEST] bfs -S ${@:check-%=%}" \
-	    ./tests/tests.sh --make="${MAKE}" --bfs="${BIN}/bfs -S ${@:check-%=%}" ${TEST_FLAGS}
+	    ./tests/tests.sh --make="${MAKE}" --bfs="bin/bfs -S ${@:check-%=%}" ${TEST_FLAGS}
 
 # Check various flags
-check-j1 check-j2 check-j3 check-s: ${BIN}/bfs ${ITEST_BINS}
+check-j1 check-j2 check-j3 check-s: bin/bfs ${ITEST_BINS}
 	+${MSG} "[TEST] bfs -${@:check-%=%}" \
-	    ./tests/tests.sh --make="${MAKE}" --bfs="${BIN}/bfs -${@:check-%=%}" ${TEST_FLAGS}
+	    ./tests/tests.sh --make="${MAKE}" --bfs="bin/bfs -${@:check-%=%}" ${TEST_FLAGS}
 
 # Run the integration tests
 integration-tests: ${INTEGRATION_TESTS}
 .PHONY: integration-tests
 
-${BIN}/tests/mksock: \
-    ${OBJ}/tests/mksock.o \
+bin/tests/mksock: \
+    obj/tests/mksock.o \
     ${LIBBFS}
 
-${BIN}/tests/xtouch: \
-    ${OBJ}/tests/xtouch.o \
+bin/tests/xtouch: \
+    obj/tests/xtouch.o \
     ${LIBBFS}
 
 # `make distcheck` configurations
@@ -183,8 +175,10 @@ DISTCHECK_CONFIG_m32 := EXTRA_CFLAGS="-m32" PKG_CONFIG_LIBDIR=/usr/lib32/pkgconf
 DISTCHECK_CONFIG_release := RELEASE=y
 
 ${DISTCHECKS}::
-	+${MAKE} -rs BUILDDIR=${BUILDDIR}/$@ config ${DISTCHECK_CONFIG_${@:distcheck-%=%}}
-	+${MAKE} -s BUILDDIR=${BUILDDIR}/$@ check TEST_FLAGS="--sudo --verbose=skipped"
+	@${MKDIR} $@
+	@+cd $@ \
+	    && ../configure ${DISTCHECK_CONFIG_${@:distcheck-%=%}} \
+	    && ${MAKE} -s check TEST_FLAGS="--sudo --verbose=skipped"
 
 ## Packaging (`make install`)
 
@@ -193,44 +187,49 @@ DEST_MANDIR := ${DESTDIR}${MANDIR}
 
 install::
 	${Q}${MKDIR} ${DEST_PREFIX}/bin
-	${MSG} "[INSTALL] bin/bfs" \
-	    ${INSTALL} -m755 ${BIN}/bfs ${DEST_PREFIX}/bin/bfs
+	${MSG} "[INST] bin/bfs" \
+	    ${INSTALL} -m755 bin/bfs ${DEST_PREFIX}/bin/bfs
 	${Q}${MKDIR} ${DEST_MANDIR}/man1
-	${MSG} "[INSTALL] man/man1/bfs.1" \
+	${MSG} "[INST] man/man1/bfs.1" \
 	    ${INSTALL} -m644 docs/bfs.1 ${DEST_MANDIR}/man1/bfs.1
 	${Q}${MKDIR} ${DEST_PREFIX}/share/bash-completion/completions
-	${MSG} "[INSTALL] completions/bfs.bash" \
+	${MSG} "[INST] completions/bfs.bash" \
 	    ${INSTALL} -m644 completions/bfs.bash ${DEST_PREFIX}/share/bash-completion/completions/bfs
 	${Q}${MKDIR} ${DEST_PREFIX}/share/zsh/site-functions
-	${MSG} "[INSTALL] completions/bfs.zsh" \
+	${MSG} "[INST] completions/bfs.zsh" \
 	    ${INSTALL} -m644 completions/bfs.zsh ${DEST_PREFIX}/share/zsh/site-functions/_bfs
 	${Q}${MKDIR} ${DEST_PREFIX}/share/fish/vendor_completions.d
-	${MSG} "[INSTALL] completions/bfs.fish" \
+		${MSG} "[INST] completions/bfs.fish" \
 	    ${INSTALL} -m644 completions/bfs.fish ${DEST_PREFIX}/share/fish/vendor_completions.d/bfs.fish
 
 uninstall::
-	${RM} ${DEST_PREFIX}/share/bash-completion/completions/bfs
-	${RM} ${DEST_PREFIX}/share/zsh/site-functions/_bfs
-	${RM} ${DEST_PREFIX}/share/fish/vendor_completions.d/bfs.fish
-	${RM} ${DEST_MANDIR}/man1/bfs.1
-	${RM} ${DEST_PREFIX}/bin/bfs
+	${MSG} "[ RM ] completions/bfs.bash" \
+	    ${RM} ${DEST_PREFIX}/share/bash-completion/completions/bfs
+	${MSG} "[ RM ] completions/bfs.zsh" \
+	    ${RM} ${DEST_PREFIX}/share/zsh/site-functions/_bfs
+	${MSG} "[ RM ] completions/bfs.fish" \
+	    ${RM} ${DEST_PREFIX}/share/fish/vendor_completions.d/bfs.fish
+	${MSG} "[ RM ] man/man1/bfs.1" \
+	    ${RM} ${DEST_MANDIR}/man1/bfs.1
+	${MSG} "[ RM ] bin/bfs" \
+	    ${RM} ${DEST_PREFIX}/bin/bfs
 
 # Check that `make install` works and `make uninstall` removes everything
 check-install::
-	+${MAKE} install DESTDIR=${BUILDDIR}/pkg
-	+${MAKE} uninstall DESTDIR=${BUILDDIR}/pkg
-	${BIN}/bfs ${BUILDDIR}/pkg -not -type d -print -exit 1
-	${RM} -r ${BUILDDIR}/pkg
+	+${MAKE} install DESTDIR=pkg
+	+${MAKE} uninstall DESTDIR=pkg
+	bin/bfs pkg -not -type d -print -exit 1
+	${RM} -r pkg
 
 ## Cleanup (`make clean`)
 
 # Clean all build products
 clean::
 	${MSG} "[ RM ] bin obj" \
-	    ${RM} -r ${BIN} ${OBJ}
+	    ${RM} -r bin obj
 
 # Clean everything, including generated files
 distclean: clean
 	${MSG} "[ RM ] gen" \
-	    ${RM} -r ${GEN} ${DISTCHECKS}
+	    ${RM} -r gen ${DISTCHECKS}
 .PHONY: distclean
diff --git a/README.md b/README.md
index b95c16b..922ebdc 100644
--- a/README.md
+++ b/README.md
@@ -333,7 +333,7 @@ Once you have the dependencies, you can build <code>bfs</code>.
 Download one of the [releases](https://github.com/tavianator/bfs/releases) or clone the [git repo](https://github.com/tavianator/bfs).
 Then run
 
-    $ make config
+    $ ./configure
     $ make
 
 This will build the `./bin/bfs` binary.
@@ -343,7 +343,7 @@ Run the test suite to make sure it works correctly:
 
 If you're interested in speed, you may want to build the release version instead:
 
-    $ make config RELEASE=y
+    $ ./configure RELEASE=y
     $ make
 
 Finally, if you want to install it globally, run
diff --git a/bench/bench.sh b/bench/bench.sh
index 1526fe5..ba46599 100644
--- a/bench/bench.sh
+++ b/bench/bench.sh
@@ -221,7 +221,7 @@ setup() {
     fi
 
     echo "Building bfs ..."
-    as-user make -s -j"$nproc" RELEASE=y
+    as-user ./configure RELEASE=y
     as-user make -s -j"$nproc" all
 
     as-user mkdir -p bench/corpus
@@ -254,8 +254,8 @@ setup() {
                 echo "Building bfs $commit ..."
                 cd "$worktree"
                 as-user git checkout -qd "$commit" --
-                if [ -e config ]; then
-                    as-user make -s -j"$nproc" config RELEASE=1
+                if [ -e configure ]; then
+                    as-user ./configure RELEASE=1
                     as-user make -s -j"$nproc"
                 else
                     as-user make -s -j"$nproc" release
diff --git a/config/cc.sh b/build/cc.sh
index 45d51ca..45d51ca 100755
--- a/config/cc.sh
+++ b/build/cc.sh
diff --git a/build/config.mk b/build/config.mk
new file mode 100644
index 0000000..153838d
--- /dev/null
+++ b/build/config.mk
@@ -0,0 +1,62 @@
+# Copyright © Tavian Barnes <tavianator@tavianator.com>
+# SPDX-License-Identifier: 0BSD
+
+# Makefile that implements `./configure`
+
+include build/prelude.mk
+include build/exports.mk
+
+# All configuration steps
+config: gen/config.mk gen/config.h
+.PHONY: config
+
+# Makefile fragments generated by `./configure`
+MKS := \
+    gen/vars.mk \
+    gen/flags.mk \
+    gen/deps.mk \
+    gen/pkgs.mk
+
+# The main configuration file, which includes the others
+gen/config.mk: ${MKS}
+	${MSG} "[ GEN] $@"
+	@printf '# %s\n' "$@" >$@
+	@printf 'include %s\n' ${.ALLSRC} >>$@
+	${VCAT} gen/config.mk
+.PHONY: gen/config.mk
+
+# Saves the configurable variables
+gen/vars.mk::
+	@${MKDIR} ${@D}
+	${MSG} "[ GEN] $@"
+	@printf '# %s\n' "$@" >$@
+	@printf 'PREFIX := %s\n' "$$XPREFIX" >>$@
+	@printf 'MANDIR := %s\n' "$$XMANDIR" >>$@
+	@printf 'OS := %s\n' "$${OS:-$$(uname)}" >>$@
+	@printf 'CC := %s\n' "$$XCC" >>$@
+	@printf 'INSTALL := %s\n' "$$XINSTALL" >>$@
+	@printf 'MKDIR := %s\n' "$$XMKDIR" >>$@
+	@printf 'PKG_CONFIG := %s\n' "$$XPKG_CONFIG" >>$@
+	@printf 'RM := %s\n' "$$XRM" >>$@
+	${VCAT} $@
+
+# Sets the build flags.  This depends on vars.mk and uses a recursive make so
+# that the default flags can depend on variables like ${OS}.
+gen/flags.mk: gen/vars.mk
+	@+${MAKE} -sf build/flags.mk $@
+.PHONY: gen/flags.mk
+
+# Check for dependency generation support
+gen/deps.mk: gen/flags.mk
+	@+${MAKE} -sf build/deps.mk $@
+.PHONY: gen/deps.mk
+
+# Auto-detect dependencies and their build flags
+gen/pkgs.mk: gen/flags.mk
+	@+${MAKE} -sf build/pkgs.mk $@
+.PHONY: gen/pkgs.mk
+
+# Compile-time feature detection
+gen/config.h: gen/config.mk
+	@+${MAKE} -sf build/header.mk $@
+.PHONY: gen/config.h
diff --git a/config/define-if.sh b/build/define-if.sh
index 059c1ac..295ead8 100755
--- a/config/define-if.sh
+++ b/build/define-if.sh
@@ -7,7 +7,7 @@
 
 set -eu
 
-SLUG="${1#config/}"
+SLUG="${1#build/}"
 SLUG="${SLUG%.c}"
 MACRO="BFS_$(printf '%s' "$SLUG" | tr '/a-z-' '_A-Z_')"
 shift
diff --git a/build/deps.mk b/build/deps.mk
new file mode 100644
index 0000000..e8dd9fb
--- /dev/null
+++ b/build/deps.mk
@@ -0,0 +1,18 @@
+# Copyright © Tavian Barnes <tavianator@tavianator.com>
+# SPDX-License-Identifier: 0BSD
+
+# Makefile that generates gen/deps.mk
+
+include build/prelude.mk
+include gen/vars.mk
+include gen/flags.mk
+include build/exports.mk
+
+gen/deps.mk::
+	${MSG} "[ GEN] $@"
+	printf '# %s\n' "$@" >$@
+	if build/cc.sh -MD -MP -MF /dev/null build/empty.c; then \
+	    printf 'DEPFLAGS := -MD -MP\n'; \
+	fi >>$@ 2>$@.log
+	${VCAT} $@
+	printf -- '-include %s\n' ${OBJS:.o=.d} >>$@
diff --git a/config/empty.c b/build/empty.c
index 4fa9a5b..4fa9a5b 100644
--- a/config/empty.c
+++ b/build/empty.c
diff --git a/config/exports.mk b/build/exports.mk
index ed19134..ed19134 100644
--- a/config/exports.mk
+++ b/build/exports.mk
diff --git a/config/flags.mk b/build/flags.mk
index 837c30f..0a44e94 100644
--- a/config/flags.mk
+++ b/build/flags.mk
@@ -3,8 +3,8 @@
 
 # Makefile that generates gen/flags.mk
 
-include config/prelude.mk
-include ${GEN}/vars.mk
+include build/prelude.mk
+include gen/vars.mk
 
 # Configurable flags
 CPPFLAGS ?=
@@ -29,7 +29,7 @@ export XLDLIBS=${LDLIBS}
 # Immutable flags
 export BFS_CPPFLAGS= \
     -Isrc \
-    -I${GEN} \
+    -Igen \
     -D__EXTENSIONS__ \
     -D_ATFILE_SOURCE \
     -D_BSD_SOURCE \
@@ -106,9 +106,9 @@ SETVAR = printf '%s := %s\n' >>$@
 # Append to a variable, if non-empty
 APPEND = append() { test -z "$$2" || printf '%s += %s\n' "$$1" "$$2" >>$@; }; append
 
-${GEN}/flags.mk::
-	${MSG} "[ GEN] ${TGT}"
-	printf '# %s\n' "${TGT}" >$@
+gen/flags.mk::
+	${MSG} "[ GEN] $@"
+	printf '# %s\n' "$@" >$@
 	${SETVAR} CPPFLAGS "$$BFS_CPPFLAGS"
 	${APPEND} CPPFLAGS "$$TSAN_CPPFLAGS"
 	${APPEND} CPPFLAGS "$$LINT_CPPFLAGS"
diff --git a/config/has/acl-get-entry.c b/build/has/acl-get-entry.c
index 3cce771..3cce771 100644
--- a/config/has/acl-get-entry.c
+++ b/build/has/acl-get-entry.c
diff --git a/config/has/acl-get-file.c b/build/has/acl-get-file.c
index 89fbf23..89fbf23 100644
--- a/config/has/acl-get-file.c
+++ b/build/has/acl-get-file.c
diff --git a/config/has/acl-get-tag-type.c b/build/has/acl-get-tag-type.c
index 2901956..2901956 100644
--- a/config/has/acl-get-tag-type.c
+++ b/build/has/acl-get-tag-type.c
diff --git a/config/has/acl-is-trivial-np.c b/build/has/acl-is-trivial-np.c
index 9ca9fc7..9ca9fc7 100644
--- a/config/has/acl-is-trivial-np.c
+++ b/build/has/acl-is-trivial-np.c
diff --git a/config/has/acl-trivial.c b/build/has/acl-trivial.c
index 7efc838..7efc838 100644
--- a/config/has/acl-trivial.c
+++ b/build/has/acl-trivial.c
diff --git a/config/has/aligned-alloc.c b/build/has/aligned-alloc.c
index 4460038..4460038 100644
--- a/config/has/aligned-alloc.c
+++ b/build/has/aligned-alloc.c
diff --git a/config/has/confstr.c b/build/has/confstr.c
index 58280b4..58280b4 100644
--- a/config/has/confstr.c
+++ b/build/has/confstr.c
diff --git a/config/has/extattr-get-file.c b/build/has/extattr-get-file.c
index ac9cf96..ac9cf96 100644
--- a/config/has/extattr-get-file.c
+++ b/build/has/extattr-get-file.c
diff --git a/config/has/extattr-get-link.c b/build/has/extattr-get-link.c
index c35be5b..c35be5b 100644
--- a/config/has/extattr-get-link.c
+++ b/build/has/extattr-get-link.c
diff --git a/config/has/extattr-list-file.c b/build/has/extattr-list-file.c
index e68a8bb..e68a8bb 100644
--- a/config/has/extattr-list-file.c
+++ b/build/has/extattr-list-file.c
diff --git a/config/has/extattr-list-link.c b/build/has/extattr-list-link.c
index 49f0ec2..49f0ec2 100644
--- a/config/has/extattr-list-link.c
+++ b/build/has/extattr-list-link.c
diff --git a/config/has/fdclosedir.c b/build/has/fdclosedir.c
index f4ad1f5..f4ad1f5 100644
--- a/config/has/fdclosedir.c
+++ b/build/has/fdclosedir.c
diff --git a/config/has/getdents.c b/build/has/getdents.c
index d0d4228..d0d4228 100644
--- a/config/has/getdents.c
+++ b/build/has/getdents.c
diff --git a/config/has/getdents64-syscall.c b/build/has/getdents64-syscall.c
index 4838c14..4838c14 100644
--- a/config/has/getdents64-syscall.c
+++ b/build/has/getdents64-syscall.c
diff --git a/config/has/getdents64.c b/build/has/getdents64.c
index 1abf36d..1abf36d 100644
--- a/config/has/getdents64.c
+++ b/build/has/getdents64.c
diff --git a/config/has/getprogname-gnu.c b/build/has/getprogname-gnu.c
index 6b97c5e..6b97c5e 100644
--- a/config/has/getprogname-gnu.c
+++ b/build/has/getprogname-gnu.c
diff --git a/config/has/getprogname.c b/build/has/getprogname.c
index 83dc8e8..83dc8e8 100644
--- a/config/has/getprogname.c
+++ b/build/has/getprogname.c
diff --git a/config/has/max-align-t.c b/build/has/max-align-t.c
index 96165ce..96165ce 100644
--- a/config/has/max-align-t.c
+++ b/build/has/max-align-t.c
diff --git a/config/has/pipe2.c b/build/has/pipe2.c
index 4cb43b5..4cb43b5 100644
--- a/config/has/pipe2.c
+++ b/build/has/pipe2.c
diff --git a/config/has/posix-spawn-addfchdir-np.c b/build/has/posix-spawn-addfchdir-np.c
index b870a53..b870a53 100644
--- a/config/has/posix-spawn-addfchdir-np.c
+++ b/build/has/posix-spawn-addfchdir-np.c
diff --git a/config/has/posix-spawn-addfchdir.c b/build/has/posix-spawn-addfchdir.c
index c52ff81..c52ff81 100644
--- a/config/has/posix-spawn-addfchdir.c
+++ b/build/has/posix-spawn-addfchdir.c
diff --git a/config/has/st-acmtim.c b/build/has/st-acmtim.c
index d687ab0..d687ab0 100644
--- a/config/has/st-acmtim.c
+++ b/build/has/st-acmtim.c
diff --git a/config/has/st-acmtimespec.c b/build/has/st-acmtimespec.c
index f747bc0..f747bc0 100644
--- a/config/has/st-acmtimespec.c
+++ b/build/has/st-acmtimespec.c
diff --git a/config/has/st-birthtim.c b/build/has/st-birthtim.c
index 4964571..4964571 100644
--- a/config/has/st-birthtim.c
+++ b/build/has/st-birthtim.c
diff --git a/config/has/st-birthtimespec.c b/build/has/st-birthtimespec.c
index 91a613f..91a613f 100644
--- a/config/has/st-birthtimespec.c
+++ b/build/has/st-birthtimespec.c
diff --git a/config/has/st-flags.c b/build/has/st-flags.c
index b1d0c32..b1d0c32 100644
--- a/config/has/st-flags.c
+++ b/build/has/st-flags.c
diff --git a/config/has/statx-syscall.c b/build/has/statx-syscall.c
index 87ec869..87ec869 100644
--- a/config/has/statx-syscall.c
+++ b/build/has/statx-syscall.c
diff --git a/config/has/statx.c b/build/has/statx.c
index 65f1674..65f1674 100644
--- a/config/has/statx.c
+++ b/build/has/statx.c
diff --git a/config/has/strerror-l.c b/build/has/strerror-l.c
index 3dcc4d7..3dcc4d7 100644
--- a/config/has/strerror-l.c
+++ b/build/has/strerror-l.c
diff --git a/config/has/strerror-r-gnu.c b/build/has/strerror-r-gnu.c
index 26ca0ee..26ca0ee 100644
--- a/config/has/strerror-r-gnu.c
+++ b/build/has/strerror-r-gnu.c
diff --git a/config/has/strerror-r-posix.c b/build/has/strerror-r-posix.c
index 41b2d30..41b2d30 100644
--- a/config/has/strerror-r-posix.c
+++ b/build/has/strerror-r-posix.c
diff --git a/config/has/tm-gmtoff.c b/build/has/tm-gmtoff.c
index 543df48..543df48 100644
--- a/config/has/tm-gmtoff.c
+++ b/build/has/tm-gmtoff.c
diff --git a/config/has/uselocale.c b/build/has/uselocale.c
index a712ff8..a712ff8 100644
--- a/config/has/uselocale.c
+++ b/build/has/uselocale.c
diff --git a/build/header.mk b/build/header.mk
new file mode 100644
index 0000000..3f77ca5
--- /dev/null
+++ b/build/header.mk
@@ -0,0 +1,69 @@
+# Copyright © Tavian Barnes <tavianator@tavianator.com>
+# SPDX-License-Identifier: 0BSD
+
+# Makefile that generates gen/config.h
+
+include build/prelude.mk
+include gen/config.mk
+include build/exports.mk
+
+# All header fragments we generate
+HEADERS := \
+    gen/has/acl-get-entry.h \
+    gen/has/acl-get-file.h \
+    gen/has/acl-get-tag-type.h \
+    gen/has/acl-is-trivial-np.h \
+    gen/has/acl-trivial.h \
+    gen/has/aligned-alloc.h \
+    gen/has/confstr.h \
+    gen/has/extattr-get-file.h \
+    gen/has/extattr-get-link.h \
+    gen/has/extattr-list-file.h \
+    gen/has/extattr-list-link.h \
+    gen/has/fdclosedir.h \
+    gen/has/getdents.h \
+    gen/has/getdents64.h \
+    gen/has/getdents64-syscall.h \
+    gen/has/getprogname.h \
+    gen/has/getprogname-gnu.h \
+    gen/has/max-align-t.h \
+    gen/has/pipe2.h \
+    gen/has/posix-spawn-addfchdir.h \
+    gen/has/posix-spawn-addfchdir-np.h \
+    gen/has/st-acmtim.h \
+    gen/has/st-acmtimespec.h \
+    gen/has/st-birthtim.h \
+    gen/has/st-birthtimespec.h \
+    gen/has/st-flags.h \
+    gen/has/statx.h \
+    gen/has/statx-syscall.h \
+    gen/has/strerror-l.h \
+    gen/has/strerror-r-gnu.h \
+    gen/has/strerror-r-posix.h \
+    gen/has/tm-gmtoff.h \
+    gen/has/uselocale.h
+
+# Previously generated by pkgs.mk
+PKG_HEADERS := ${ALL_PKGS:%=gen/use/%.h}
+
+gen/config.h: ${PKG_HEADERS} ${HEADERS}
+	${MSG} "[ GEN] $@"
+	printf '// %s\n' "$@" >$@
+	printf '#ifndef BFS_CONFIG_H\n' >>$@
+	printf '#define BFS_CONFIG_H\n' >>$@
+	cat ${.ALLSRC} >>$@
+	printf '#endif // BFS_CONFIG_H\n' >>$@
+	cat ${.ALLSRC:%=%.log} >$@.log
+	${VCAT} $@
+.PHONY: gen/config.h
+
+# The short name of the config test
+SLUG = ${@:gen/%.h=%}
+
+${HEADERS}::
+	${MKDIR} ${@D}
+	if build/define-if.sh ${SLUG} build/cc.sh build/${SLUG}.c >$@ 2>$@.log; then \
+	    test "${IS_V}" || printf '[ CC ] %-${MSG_WIDTH}s  ✔\n' ${SLUG}.c; \
+	else \
+	    test "${IS_V}" || printf '[ CC ] %-${MSG_WIDTH}s  ✘\n' ${SLUG}.c; \
+	fi
diff --git a/config/pkgconf.sh b/build/pkgconf.sh
index 365ed61..96e4bf1 100755
--- a/config/pkgconf.sh
+++ b/build/pkgconf.sh
@@ -41,7 +41,7 @@ if [ -z "$MODE" ]; then
         CFLAGS=$("$0" --cflags "$LIB") || exit 1
         LDFLAGS=$("$0" --ldflags "$LIB") || exit 1
         LDLIBS=$("$0" --ldlibs "$LIB") || exit 1
-        config/cc.sh $CFLAGS $LDFLAGS config/use/$LIB.c $LDLIBS || exit 1
+        build/cc.sh $CFLAGS $LDFLAGS build/use/$LIB.c $LDLIBS || exit 1
     done
 fi
 
diff --git a/build/pkgs.mk b/build/pkgs.mk
new file mode 100644
index 0000000..51ca664
--- /dev/null
+++ b/build/pkgs.mk
@@ -0,0 +1,36 @@
+# Copyright © Tavian Barnes <tavianator@tavianator.com>
+# SPDX-License-Identifier: 0BSD
+
+# Makefile that generates gen/pkgs.mk
+
+include build/prelude.mk
+include gen/vars.mk
+include gen/flags.mk
+include build/exports.mk
+
+HEADERS := ${ALL_PKGS:%=gen/use/%.h}
+
+gen/pkgs.mk: ${HEADERS}
+	${MSG} "[ GEN] $@"
+	printf '# %s\n' "$@" >$@
+	gen() { \
+	    printf 'PKGS := %s\n' "$$*"; \
+	    printf 'CFLAGS += %s\n' "$$(build/pkgconf.sh --cflags "$$@")"; \
+	    printf 'LDFLAGS += %s\n' "$$(build/pkgconf.sh --ldflags "$$@")"; \
+	    printf 'LDLIBS := %s $${LDLIBS}\n' "$$(build/pkgconf.sh --ldlibs "$$@")"; \
+	}; \
+	gen $$(grep -l ' true$$' ${.ALLSRC} | sed 's|.*/\(.*\)\.h|\1|') >>$@
+	${VCAT} $@
+
+.PHONY: gen/pkgs.mk
+
+# Convert gen/use/foo.h to foo
+PKG = ${@:gen/use/%.h=%}
+
+${HEADERS}::
+	${MKDIR} ${@D}
+	if build/define-if.sh use/${PKG} build/pkgconf.sh ${PKG} >$@ 2>$@.log; then \
+	    test "${IS_V}" || printf '[ CC ] %-${MSG_WIDTH}s  ✔\n' use/${PKG}.c; \
+	else \
+	    test "${IS_V}" || printf '[ CC ] %-${MSG_WIDTH}s  ✘\n' use/${PKG}.c; \
+	fi
diff --git a/config/prelude.mk b/build/prelude.mk
index 109e60c..b235fa9 100644
--- a/config/prelude.mk
+++ b/build/prelude.mk
@@ -15,38 +15,12 @@
 # BSD's ${.ALLSRC} to GNU.
 .ALLSRC ?= $^
 
-# For out-of-tree builds, e.g.
-#
-#     $ make config BUILDDIR=/path/to/build/dir
-#     $ make BUILDDIR=/path/to/build/dir
-BUILDDIR ?= .
-
-# Shorthand for build subdirectories
-BIN := ${BUILDDIR}/bin
-GEN := ${BUILDDIR}/gen
-OBJ := ${BUILDDIR}/obj
-
-# GNU make strips a leading ./ from target names, so do the same for BSD make
-BIN := ${BIN:./%=%}
-GEN := ${GEN:./%=%}
-OBJ := ${OBJ:./%=%}
-
-# The configuration file generated by `make config`
-CONFIG := ${GEN}/config.mk
-
 # Installation paths
 DESTDIR ?=
 PREFIX ?= /usr
 MANDIR ?= ${PREFIX}/share/man
 
-# GNU make supports `export VAR`, but BSD make requires `export VAR=value`.
-# Sadly, GNU make gives a recursion error on `export VAR=${VAR}`.
-_BUILDDIR := ${BUILDDIR}
-export BUILDDIR=${_BUILDDIR}
-
-# Configurable executables; can be overridden with
-#
-#     $ make config CC=clang
+# Configurable executables
 CC ?= cc
 INSTALL ?= install
 MKDIR ?= mkdir -p
@@ -79,9 +53,6 @@ IS_V := ${TRUTHY,${V}}
 Q, := @
 Q  := ${Q,${IS_V}}
 
-# The current target, with ${BUILDDIR} stripped for shorter messages
-TGT = ${@:${BUILDDIR}/%=%}
-
 # Show full commands with `make V=1`, otherwise short summaries
 MSG = @msg() { \
           MSG="$$1"; \
@@ -109,54 +80,54 @@ ALL_PKGS := \
     liburing \
     oniguruma
 
-# List all object files here, as they're needed by both `make config` and `make`
+# List all object files here, as they're needed by both `./configure` and `make`
 
 # All object files except the entry point
 LIBBFS := \
-    ${OBJ}/src/alloc.o \
-    ${OBJ}/src/bar.o \
-    ${OBJ}/src/bfstd.o \
-    ${OBJ}/src/bftw.o \
-    ${OBJ}/src/color.o \
-    ${OBJ}/src/ctx.o \
-    ${OBJ}/src/diag.o \
-    ${OBJ}/src/dir.o \
-    ${OBJ}/src/dstring.o \
-    ${OBJ}/src/eval.o \
-    ${OBJ}/src/exec.o \
-    ${OBJ}/src/expr.o \
-    ${OBJ}/src/fsade.o \
-    ${OBJ}/src/ioq.o \
-    ${OBJ}/src/mtab.o \
-    ${OBJ}/src/opt.o \
-    ${OBJ}/src/parse.o \
-    ${OBJ}/src/printf.o \
-    ${OBJ}/src/pwcache.o \
-    ${OBJ}/src/stat.o \
-    ${OBJ}/src/thread.o \
-    ${OBJ}/src/trie.o \
-    ${OBJ}/src/typo.o \
-    ${OBJ}/src/xregex.o \
-    ${OBJ}/src/xspawn.o \
-    ${OBJ}/src/xtime.o \
-    ${OBJ}/gen/version.o
+    obj/src/alloc.o \
+    obj/src/bar.o \
+    obj/src/bfstd.o \
+    obj/src/bftw.o \
+    obj/src/color.o \
+    obj/src/ctx.o \
+    obj/src/diag.o \
+    obj/src/dir.o \
+    obj/src/dstring.o \
+    obj/src/eval.o \
+    obj/src/exec.o \
+    obj/src/expr.o \
+    obj/src/fsade.o \
+    obj/src/ioq.o \
+    obj/src/mtab.o \
+    obj/src/opt.o \
+    obj/src/parse.o \
+    obj/src/printf.o \
+    obj/src/pwcache.o \
+    obj/src/stat.o \
+    obj/src/thread.o \
+    obj/src/trie.o \
+    obj/src/typo.o \
+    obj/src/xregex.o \
+    obj/src/xspawn.o \
+    obj/src/xtime.o \
+    obj/gen/version.o
 
 # Unit test objects
 UNIT_OBJS := \
-    ${OBJ}/tests/alloc.o \
-    ${OBJ}/tests/bfstd.o \
-    ${OBJ}/tests/bit.o \
-    ${OBJ}/tests/ioq.o \
-    ${OBJ}/tests/main.o \
-    ${OBJ}/tests/trie.o \
-    ${OBJ}/tests/xspawn.o \
-    ${OBJ}/tests/xtime.o
+    obj/tests/alloc.o \
+    obj/tests/bfstd.o \
+    obj/tests/bit.o \
+    obj/tests/ioq.o \
+    obj/tests/main.o \
+    obj/tests/trie.o \
+    obj/tests/xspawn.o \
+    obj/tests/xtime.o
 
 # All object files
 OBJS := \
-    ${OBJ}/src/main.o \
-    ${OBJ}/tests/mksock.o \
-    ${OBJ}/tests/xspawnee.o \
-    ${OBJ}/tests/xtouch.o \
+    obj/src/main.o \
+    obj/tests/mksock.o \
+    obj/tests/xspawnee.o \
+    obj/tests/xtouch.o \
     ${LIBBFS} \
     ${UNIT_OBJS}
diff --git a/config/use/libacl.c b/build/use/libacl.c
index de1fe50..de1fe50 100644
--- a/config/use/libacl.c
+++ b/build/use/libacl.c
diff --git a/config/use/libcap.c b/build/use/libcap.c
index 58e832c..58e832c 100644
--- a/config/use/libcap.c
+++ b/build/use/libcap.c
diff --git a/config/use/libselinux.c b/build/use/libselinux.c
index bca409d..bca409d 100644
--- a/config/use/libselinux.c
+++ b/build/use/libselinux.c
diff --git a/config/use/liburing.c b/build/use/liburing.c
index bea499a..bea499a 100644
--- a/config/use/liburing.c
+++ b/build/use/liburing.c
diff --git a/config/use/oniguruma.c b/build/use/oniguruma.c
index cb17596..cb17596 100644
--- a/config/use/oniguruma.c
+++ b/build/use/oniguruma.c
diff --git a/config/config.mk b/config/config.mk
deleted file mode 100644
index 280c6ac..0000000
--- a/config/config.mk
+++ /dev/null
@@ -1,63 +0,0 @@
-# Copyright © Tavian Barnes <tavianator@tavianator.com>
-# SPDX-License-Identifier: 0BSD
-
-# Makefile fragment that implements `make config`
-
-include config/prelude.mk
-include config/exports.mk
-
-# All configuration steps
-config: ${CONFIG} ${GEN}/config.h
-.PHONY: config
-
-# Makefile fragments generated by `make config`
-MKS := \
-    ${GEN}/vars.mk \
-    ${GEN}/flags.mk \
-    ${GEN}/deps.mk \
-    ${GEN}/pkgs.mk
-
-# The main configuration file, which includes the others
-${CONFIG}: ${MKS}
-	${MSG} "[ GEN] ${TGT}"
-	@printf '# %s\n' "${TGT}" >$@
-	@printf 'include $${GEN}/%s\n' ${MKS:${GEN}/%=%} >>$@
-	${VCAT} ${CONFIG}
-.PHONY: ${CONFIG}
-
-# Saves the configurable variables
-${GEN}/vars.mk::
-	@${MKDIR} ${@D}
-	${MSG} "[ GEN] ${TGT}"
-	@printf '# %s\n' "${TGT}" >$@
-	@printf 'PREFIX := %s\n' "$$XPREFIX" >>$@
-	@printf 'MANDIR := %s\n' "$$XMANDIR" >>$@
-	@printf 'OS := %s\n' "$${OS:-$$(uname)}" >>$@
-	@printf 'CC := %s\n' "$$XCC" >>$@
-	@printf 'INSTALL := %s\n' "$$XINSTALL" >>$@
-	@printf 'MKDIR := %s\n' "$$XMKDIR" >>$@
-	@printf 'PKG_CONFIG := %s\n' "$$XPKG_CONFIG" >>$@
-	@printf 'RM := %s\n' "$$XRM" >>$@
-	@printf 'PKGS :=\n' >>$@
-	${VCAT} $@
-
-# Sets the build flags.  This depends on vars.mk and uses a recursive make so
-# that the default flags can depend on variables like ${OS}.
-${GEN}/flags.mk: ${GEN}/vars.mk
-	@+${MAKE} -sf config/flags.mk $@
-.PHONY: ${GEN}/flags.mk
-
-# Check for dependency generation support
-${GEN}/deps.mk: ${GEN}/flags.mk
-	@+${MAKE} -sf config/deps.mk $@
-.PHONY: ${GEN}/deps.mk
-
-# Auto-detect dependencies and their build flags
-${GEN}/pkgs.mk: ${GEN}/flags.mk
-	@+${MAKE} -sf config/pkgs.mk $@
-.PHONY: ${GEN}/pkgs.mk
-
-# Compile-time feature detection
-${GEN}/config.h: ${CONFIG}
-	@+${MAKE} -sf config/header.mk $@
-.PHONY: ${GEN}/config.h
diff --git a/config/deps.mk b/config/deps.mk
deleted file mode 100644
index ac394a5..0000000
--- a/config/deps.mk
+++ /dev/null
@@ -1,18 +0,0 @@
-# Copyright © Tavian Barnes <tavianator@tavianator.com>
-# SPDX-License-Identifier: 0BSD
-
-# Makefile that generates gen/deps.mk
-
-include config/prelude.mk
-include ${GEN}/vars.mk
-include ${GEN}/flags.mk
-include config/exports.mk
-
-${GEN}/deps.mk::
-	${MSG} "[ GEN] ${TGT}"
-	printf '# %s\n' "${TGT}" >$@
-	if config/cc.sh -MD -MP -MF /dev/null config/empty.c; then \
-	    printf 'DEPFLAGS = -MD -MP\n'; \
-	fi >>$@ 2>$@.log
-	${VCAT} $@
-	printf -- '-include %s\n' ${OBJS:.o=.d} >>$@
diff --git a/config/header.mk b/config/header.mk
deleted file mode 100644
index 50ad5e8..0000000
--- a/config/header.mk
+++ /dev/null
@@ -1,69 +0,0 @@
-# Copyright © Tavian Barnes <tavianator@tavianator.com>
-# SPDX-License-Identifier: 0BSD
-
-# Makefile that generates gen/config.h
-
-include config/prelude.mk
-include ${GEN}/config.mk
-include config/exports.mk
-
-# All header fragments we generate
-HEADERS := \
-    ${GEN}/has/acl-get-entry.h \
-    ${GEN}/has/acl-get-file.h \
-    ${GEN}/has/acl-get-tag-type.h \
-    ${GEN}/has/acl-is-trivial-np.h \
-    ${GEN}/has/acl-trivial.h \
-    ${GEN}/has/aligned-alloc.h \
-    ${GEN}/has/confstr.h \
-    ${GEN}/has/extattr-get-file.h \
-    ${GEN}/has/extattr-get-link.h \
-    ${GEN}/has/extattr-list-file.h \
-    ${GEN}/has/extattr-list-link.h \
-    ${GEN}/has/fdclosedir.h \
-    ${GEN}/has/getdents.h \
-    ${GEN}/has/getdents64.h \
-    ${GEN}/has/getdents64-syscall.h \
-    ${GEN}/has/getprogname.h \
-    ${GEN}/has/getprogname-gnu.h \
-    ${GEN}/has/max-align-t.h \
-    ${GEN}/has/pipe2.h \
-    ${GEN}/has/posix-spawn-addfchdir.h \
-    ${GEN}/has/posix-spawn-addfchdir-np.h \
-    ${GEN}/has/st-acmtim.h \
-    ${GEN}/has/st-acmtimespec.h \
-    ${GEN}/has/st-birthtim.h \
-    ${GEN}/has/st-birthtimespec.h \
-    ${GEN}/has/st-flags.h \
-    ${GEN}/has/statx.h \
-    ${GEN}/has/statx-syscall.h \
-    ${GEN}/has/strerror-l.h \
-    ${GEN}/has/strerror-r-gnu.h \
-    ${GEN}/has/strerror-r-posix.h \
-    ${GEN}/has/tm-gmtoff.h \
-    ${GEN}/has/uselocale.h
-
-# Previously generated by pkgs.mk
-PKG_HEADERS := ${ALL_PKGS:%=${GEN}/use/%.h}
-
-${GEN}/config.h: ${PKG_HEADERS} ${HEADERS}
-	${MSG} "[ GEN] ${TGT}"
-	printf '// %s\n' "${TGT}" >$@
-	printf '#ifndef BFS_CONFIG_H\n' >>$@
-	printf '#define BFS_CONFIG_H\n' >>$@
-	cat ${.ALLSRC} >>$@
-	printf '#endif // BFS_CONFIG_H\n' >>$@
-	cat ${.ALLSRC:%=%.log} >$@.log
-	${VCAT} $@
-.PHONY: ${GEN}/config.h
-
-# The short name of the config test
-SLUG = ${@:${GEN}/%.h=%}
-
-${HEADERS}::
-	${MKDIR} ${@D}
-	if config/define-if.sh ${SLUG} config/cc.sh config/${SLUG}.c >$@ 2>$@.log; then \
-	    test "${IS_V}" || printf '[ CC ] %-${MSG_WIDTH}s  ✔\n' ${SLUG}.c; \
-	else \
-	    test "${IS_V}" || printf '[ CC ] %-${MSG_WIDTH}s  ✘\n' ${SLUG}.c; \
-	fi
diff --git a/config/pkgs.mk b/config/pkgs.mk
deleted file mode 100644
index 898f262..0000000
--- a/config/pkgs.mk
+++ /dev/null
@@ -1,36 +0,0 @@
-# Copyright © Tavian Barnes <tavianator@tavianator.com>
-# SPDX-License-Identifier: 0BSD
-
-# Makefile that generates gen/pkgs.mk
-
-include config/prelude.mk
-include ${GEN}/vars.mk
-include ${GEN}/flags.mk
-include config/exports.mk
-
-HEADERS := ${ALL_PKGS:%=${GEN}/use/%.h}
-
-${GEN}/pkgs.mk: ${HEADERS}
-	${MSG} "[ GEN] ${TGT}"
-	printf '# %s\n' "${TGT}" >$@
-	gen() { \
-	    printf 'PKGS := %s\n' "$$*"; \
-	    printf 'CFLAGS += %s\n' "$$(config/pkgconf.sh --cflags "$$@")"; \
-	    printf 'LDFLAGS += %s\n' "$$(config/pkgconf.sh --ldflags "$$@")"; \
-	    printf 'LDLIBS := %s $${LDLIBS}\n' "$$(config/pkgconf.sh --ldlibs "$$@")"; \
-	}; \
-	gen $$(grep -l ' true$$' ${.ALLSRC} | sed 's|.*/\(.*\)\.h|\1|') >>$@
-	${VCAT} $@
-
-.PHONY: ${GEN}/pkgs.mk
-
-# Convert ${GEN}/use/foo.h to foo
-PKG = ${@:${GEN}/use/%.h=%}
-
-${HEADERS}::
-	${MKDIR} ${@D}
-	if config/define-if.sh use/${PKG} config/pkgconf.sh ${PKG} >$@ 2>$@.log; then \
-	    test "${IS_V}" || printf '[ CC ] %-${MSG_WIDTH}s  ✔\n' use/${PKG}.c; \
-	else \
-	    test "${IS_V}" || printf '[ CC ] %-${MSG_WIDTH}s  ✘\n' use/${PKG}.c; \
-	fi
diff --git a/configure b/configure
new file mode 100755
index 0000000..6920212
--- /dev/null
+++ b/configure
@@ -0,0 +1,98 @@
+#!/bin/sh
+
+# Copyright © Tavian Barnes <tavianator@tavianator.com>
+# SPDX-License-Identifier: 0BSD
+
+# bfs build configuration script
+
+set -eu
+
+help() {
+    cat <<EOF
+Usage: $0 [-j<N>] [CC=...] [CFLAGS=...] [...]
+
+Compiler configuration:
+
+  CC
+      The C compiler to use
+  CPPFLAGS
+      C preprocessor flags
+  CFLAGS
+      C compiler flags
+  LDFLAGS
+      Linker flags
+  LDLIBS
+      Dynamic libraries to link
+
+  EXTRA_CPPFLAGS
+  EXTRA_CFLAGS
+  EXTRA_LDFLAGS
+  EXTRA_LDLIBS
+      Adds to the default flags, instead of replacing them
+
+Build profiles:
+
+  RELEASE=y
+      Enable optimizations, disable assertions
+  ASAN=y
+  LSAN=y
+  MSAN=y
+  TSAN=y
+  UBSAN=y
+      Enable sanitizers
+  GCOV=y
+      Enable code coverage instrumentation
+
+External dependencies:
+
+  PKG_CONFIG
+      The pkg-config binary to use
+
+  USE_LIBACL=[y|n]
+  USE_LIBCAP=[y|n]
+  USE_LIBSELINUX=[y|n]
+  USE_LIBURIG=[y|n]
+  USE_ONIGURUMA=[y|n]
+      Enable or disable external dependencies
+
+Packaging configuration:
+
+  PREFIX
+      Set the installation prefix (default: /usr)
+  MANDIR
+      Set the man page directory (default: \$PREFIX/share/man)
+EOF
+}
+
+for arg; do
+    case "$arg" in
+        --help)
+            help
+            exit 0
+            ;;
+        -*|*=*)
+            continue # make flag (-j2) or variable (CC=clang)
+            ;;
+        *)
+            printf 'error: Unrecognized option "%s"\n\n' "$arg" >&2
+            help >&2
+            exit 1
+            ;;
+    esac
+done
+
+# Get the relative path to the source tree based on how the script was run
+DIR=$(dirname -- "$0")
+
+# Set up symbolic links for out-of-tree builds
+for f in Makefile build completions docs src tests; do
+    test -e "$f" || ln -s "$DIR/$f" "$f"
+done
+
+# Infer -jN unless MAKEFLAGS is set
+j=
+if ! [ "${MAKEFLAGS+y}" ]; then
+    j="-j$({ nproc || sysctl -n hw.ncpu || getconf _NPROCESSORS_ONLN || echo 1; } 2>/dev/null)"
+fi
+
+${MAKE:-make} $j -rsf build/config.mk "$@"
diff --git a/docs/BUILDING.md b/docs/BUILDING.md
index 4ed139c..db5d721 100644
--- a/docs/BUILDING.md
+++ b/docs/BUILDING.md
@@ -4,10 +4,9 @@ Building `bfs`
 Compiling
 ---------
 
-`bfs` uses [GNU Make](https://www.gnu.org/software/make/) as its build system.
 A simple invocation of
 
-    $ make config
+    $ ./configure
     $ make
 
 should build `bfs` successfully.
@@ -18,7 +17,6 @@ For example, to use all your cores, run `make -j$(nproc)`.
 
 | Command          | Description                                                   |
 |------------------|---------------------------------------------------------------|
-| `make config`    | Configures the build system                                   |
 | `make`           | Builds just the `bfs` binary                                  |
 | `make all`       | Builds everything, including the tests (but doesn't run them) |
 | `make check`     | Builds everything, and runs the tests                         |
@@ -33,13 +31,13 @@ The configuration system provides a few shorthand flags for handy configurations
 
 | Command                 | Description                                                 |
 |-------------------------|-------------------------------------------------------------|
-| `make config RELEASE=y` | Build `bfs` with optimizations, LTO, and without assertions |
-| `make config ASAN=y`    | Enable [AddressSanitizer]                                   |
-| `make config LSAN=y`    | Enable [LeakSanitizer]                                      |
-| `make config MSAN=y`    | Enable [MemorySanitizer]                                    |
-| `make config TSAN=y`    | Enable [ThreadSanitizer]                                    |
-| `make config UBSAN=y`   | Enable [UndefinedBehaviorSanitizer]                         |
-| `make config GCOV=y`    | Enable [code coverage]                                      |
+| `./configure RELEASE=y` | Build `bfs` with optimizations, LTO, and without assertions |
+| `./configure ASAN=y`    | Enable [AddressSanitizer]                                   |
+| `./configure LSAN=y`    | Enable [LeakSanitizer]                                      |
+| `./configure MSAN=y`    | Enable [MemorySanitizer]                                    |
+| `./configure TSAN=y`    | Enable [ThreadSanitizer]                                    |
+| `./configure UBSAN=y`   | Enable [UndefinedBehaviorSanitizer]                         |
+| `./configure GCOV=y`    | Enable [code coverage]                                      |
 
 [AddressSanitizer]: https://github.com/google/sanitizers/wiki/AddressSanitizer
 [LeakSanitizer]: https://github.com/google/sanitizers/wiki/AddressSanitizerLeakSanitizer#stand-alone-mode
@@ -48,21 +46,20 @@ The configuration system provides a few shorthand flags for handy configurations
 [UndefinedBehaviorSanitizer]: https://clang.llvm.org/docs/UndefinedBehaviorSanitizer.html
 [code coverage]: https://gcc.gnu.org/onlinedocs/gcc/Gcov.html
 
-You can combine multiple profiles (e.g. `make config ASAN=y UBSAN=y`), but not all of them will work together.
+You can combine multiple profiles (e.g. `./configure ASAN=y UBSAN=y`), but not all of them will work together.
 
 ### Flags
 
-Other flags can be specified on the `make config` command line or in the environment.
+Other flags can be specified on the `./configure` command line or in the environment.
 Here are some of the common ones; check the [`Makefile`](/Makefile) for more.
 
 | Flag                                | Description                                        |
 |-------------------------------------|----------------------------------------------------|
-| `CC`                                | The C compiler to use, e.g. `make config CC=clang` |
+| `CC`                                | The C compiler to use, e.g. `./configure CC=clang` |
 | `CFLAGS`<br>`EXTRA_CFLAGS`          | Override/add to the default compiler flags         |
 | `LDFLAGS`<br>`EXTRA_LDFLAGS`        | Override/add to the linker flags                   |
 | `USE_LIBACL`<br>`USE_LIBCAP`<br>... | Enable/disable [optional dependencies]             |
 | `TEST_FLAGS`                        | `tests.sh` flags for `make check`                  |
-| `BUILDDIR`                          | The build output directory (default: `.`)          |
 | `DESTDIR`                           | The root directory for `make install`              |
 | `PREFIX`                            | The installation prefix (default: `/usr`)          |
 | `MANDIR`                            | The man page installation directory                |
@@ -72,9 +69,9 @@ Here are some of the common ones; check the [`Makefile`](/Makefile) for more.
 ### Dependencies
 
 `bfs` depends on some system libraries for some of its features.
-These dependencies are optional, and can be turned off in `make config` if necessary by setting the appropriate variable to `n` (e.g. `make config USE_ONIGURUMA=n`).
+These dependencies are optional, and can be turned off in `./configure` if necessary by setting the appropriate variable to `n` (e.g. `./configure USE_ONIGURUMA=n`).
 
-| Dependency   | Platforms  | `make config` flag |
+| Dependency   | Platforms  | `./configure` flag |
 |--------------|------------|--------------------|
 | [libacl]     | Linux only | `USE_LIBACL`       |
 | [libcap]     | Linux only | `USE_LIBCAP`       |
@@ -90,22 +87,28 @@ These dependencies are optional, and can be turned off in `make config` if neces
 
 ### Dependency tracking
 
-The build system automatically tracks header dependencies with the `-M` family of compiler options (see `DEPFLAGS` in the [`Makefile`](/Makefile)).
+The build system automatically tracks header dependencies with the `-M` family of compiler options (see `DEPFLAGS` in [`build/deps.mk`](/build/deps.mk)).
 So if you edit a header file, `make` will rebuild the necessary object files ensuring they don't go out of sync.
 
 We also add a dependency on the current configuration, so you can change configurations and rebuild without having to `make clean`.
-
-We go one step further than most build systems by tracking the flags that were used for the previous compilation.
-That means you can change configurations without having to `make clean`.
 For example,
 
-    $ make config
+    $ ./configure
     $ make
-    $ make config RELEASE=y
+    $ ./configure RELEASE=y
     $ make
 
 will build the project in debug mode and then rebuild it in release mode.
 
+### Out-of-tree builds
+
+You can set up an out-of-tree build by running the `configure` script from another directory, for example:
+
+    $ mkdir out
+    $ cd out
+    $ ../configure
+    $ make
+
 
 Testing
 -------
diff --git a/tests/util.sh b/tests/util.sh
index 7dba9fb..3969db5 100644
--- a/tests/util.sh
+++ b/tests/util.sh
@@ -12,12 +12,9 @@ _realpath() (
 )
 
 # Globals
-TESTS=$(_realpath "$TESTS")
-if [ "${BUILDDIR-}" ]; then
-    BIN=$(_realpath "$BUILDDIR/bin")
-else
-    BIN=$(_realpath "$TESTS/../bin")
-fi
+ROOT=$(_realpath "$(dirname -- "$TESTS")")
+TESTS="$ROOT/tests"
+BIN="$ROOT/bin"
 MKSOCK="$BIN/tests/mksock"
 XTOUCH="$BIN/tests/xtouch"
 UNAME=$(uname)
diff --git a/tests/xspawn.c b/tests/xspawn.c
index 7362aa5..785ea48 100644
--- a/tests/xspawn.c
+++ b/tests/xspawn.c
@@ -64,27 +64,20 @@ static bool check_use_path(bool use_posix) {
 		spawn.flags &= ~BFS_SPAWN_USE_POSIX;
 	}
 
-	const char *builddir = getenv("BUILDDIR");
-	dchar *bin = dstrprintf("%s/bin", builddir ? builddir : ".");
-	ret &= bfs_pcheck(bin, "dstrprintf()");
-	if (!ret) {
-		goto destroy;
-	}
-
-	ret &= bfs_pcheck(bfs_spawn_addopen(&spawn, 10, bin, O_RDONLY | O_DIRECTORY, 0) == 0);
+	ret &= bfs_pcheck(bfs_spawn_addopen(&spawn, 10, "bin", O_RDONLY | O_DIRECTORY, 0) == 0);
 	ret &= bfs_pcheck(bfs_spawn_adddup2(&spawn, 10, 11) == 0);
 	ret &= bfs_pcheck(bfs_spawn_addclose(&spawn, 10) == 0);
 	ret &= bfs_pcheck(bfs_spawn_addfchdir(&spawn, 11) == 0);
 	ret &= bfs_pcheck(bfs_spawn_addclose(&spawn, 11) == 0);
 	if (!ret) {
-		goto bin;
+		goto destroy;
 	}
 
 	// Check that $PATH is resolved in the parent's environment
 	char **envp;
 	ret &= bfs_pcheck(envp = envdup());
 	if (!ret) {
-		goto bin;
+		goto destroy;
 	}
 
 	// Check that $PATH is resolved after the file actions
@@ -138,8 +131,6 @@ env:
 		free(*var);
 	}
 	free(envp);
-bin:
-	dstrfree(bin);
 destroy:
 	ret &= bfs_pcheck(bfs_spawn_destroy(&spawn) == 0);
 out: