build: track config args

Record the ./configure arguments used and make them user-visible.

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
Acked-by: Paul Jakma <paul@jakma.org>
diff --git a/configure.ac b/configure.ac
index 33a1cd0..5a3f816 100755
--- a/configure.ac
+++ b/configure.ac
@@ -8,6 +8,8 @@
 AC_PREREQ(2.53)
 
 AC_INIT(Quagga, 0.99.24-rc1, [https://bugzilla.quagga.net])
+CONFIG_ARGS="$*"
+AC_SUBST(CONFIG_ARGS)
 AC_CONFIG_SRCDIR(lib/zebra.h)
 AC_CONFIG_MACRO_DIR([m4])