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/libostree/ostree-core.c

@@ -20,9 +20,6 @@
  * Author: Colin Walters <walters@verbum.org>
  */
 
-/* for mkdtemp */
-#define _GNU_SOURCE
-
 #include "config.h"
 
 #include "ostree.h"

src/libostree/ostree-diff.c

@@ -20,8 +20,6 @@
  * Author: Colin Walters <walters@verbum.org>
  */
 
-#define _GNU_SOURCE
-
 #include "config.h"
 
 #include "ostree.h"

src/libostree/ostree-repo-traverse.c

@@ -20,8 +20,6 @@
  * Author: Colin Walters <walters@verbum.org>
  */
 
-#define _GNU_SOURCE
-
 #include "config.h"
 
 #include "ostree.h"