Use AC_USE_SYSTEM_EXTENSIONS to get _GNU_SOURCE, _XOPEN_SOURCE etc.

Similar to Bug #684123 in GLib.

Bug: https://bugzilla.gnome.org/show_bug.cgi?id=641638
Bug-Debian: https://bugs.debian.org/756476
Reviewed-by: Matthias Clasen

gtk/gtksearchenginesimple.c

@@ -21,14 +21,6 @@
 
 #include "config.h"
 
-/* these must be defined even when HAVE_GNU_FTW is not defined
- * because (really) old versions of GNU libc have ftw.h but do
- * export ftw() and friends only if _XOPEN_SOURCE and _GNU_SOURCE
- * are defined. see bug #444097.
- */
-#define _XOPEN_SOURCE 600
-#define _GNU_SOURCE
-
 #ifdef HAVE_FTW_H
 #include <ftw.h>
 #endif