autogen.sh: Restore passing --enable-maintainer-mode to configure.

Otherwise build system files aren't automagically regenerated after updating
from Git.

Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
diff --git a/autogen.sh b/autogen.sh
index 30d679f..3f190ba 100755
--- a/autogen.sh
+++ b/autogen.sh
@@ -3,4 +3,4 @@
 test -n "$srcdir" || srcdir=`dirname "$0"`
 test -n "$srcdir" || srcdir=.
 autoreconf --force --install --verbose "$srcdir"
-test -n "$NOCONFIGURE" || "$srcdir/configure" "$@"
+test -n "$NOCONFIGURE" || "$srcdir/configure" --enable-maintainer-mode "$@"