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.

1 files changed, 8 insertions, 0 deletions

diff --git a/build/has/aligned-alloc.c b/build/has/aligned-alloc.c
new file mode 100644
index 0000000..4460038
--- /dev/null
+++ b/build/has/aligned-alloc.c
@@ -0,0 +1,8 @@
+// Copyright © Tavian Barnes <tavianator@tavianator.com>
+// SPDX-License-Identifier: 0BSD
+
+#include <stdlib.h>
+
+int main(void) {
+	return !aligned_alloc(_Alignof(void *), sizeof(void *));
+}