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, 9 insertions, 0 deletions

diff --git a/build/has/getprogname-gnu.c b/build/has/getprogname-gnu.c
new file mode 100644
index 0000000..6b97c5e
--- /dev/null
+++ b/build/has/getprogname-gnu.c
@@ -0,0 +1,9 @@
+// Copyright © Tavian Barnes <tavianator@tavianator.com>
+// SPDX-License-Identifier: 0BSD
+
+#include <errno.h>
+
+int main(void) {
+	const char *str = program_invocation_short_name;
+	return str[0];
+}