Use AC_USE_SYSTEM_EXTENSIONS instead of #define _GNU_SOURCE

It's less copy/paste, works everywhere, etc.

Also fix some missing #include "config.h".

src/switchroot/ostree-remount.c

@@ -20,7 +20,7 @@
  * Author: Colin Walters <walters@verbum.org>
  */
 
-#define _GNU_SOURCE
+#include "config.h"
 
 #include <string.h>
 #include <stdio.h>