re-add -g CFLAGS with --enable-debug

after commit 8feccaed78, -g was no longer added with
--enable-debug.

So if CFLAGS was set (if unset, it defaults to -g -O2) and didn't contain
-g, we ended with no debug symbols..

Signed-off-by: Xavier Chantry <shiningxc@gmail.com>
Signed-off-by: Dan McGee <dan@archlinux.org>

configure.ac

@@ -284,7 +284,7 @@ if test "x$debug" = "xyes" ; then
 	# Check for -fstack-protector availability
 	GCC_STACK_PROTECT_LIB
 	GCC_STACK_PROTECT_CC
-	CFLAGS="$CFLAGS -Wall -Werror"
+	CFLAGS="$CFLAGS -g -Wall -Werror"
 else
 	AC_MSG_RESULT(no)
 	CFLAGS="$CFLAGS -Wall"