Add _XOPEN_SOURCE display.c for gethostname()

We shouldn't need _GNU_SOURCE in delete.c anymore either.

https://bugzilla.gnome.org/show_bug.cgi?id=620860

src/core/delete.c

@@ -22,8 +22,7 @@
  * 02111-1307, USA.
  */
 
-#define _GNU_SOURCE
-#define _SVID_SOURCE /* for gethostname() */
+#define _XOPEN_SOURCE /* for kill() */
 
 #include <config.h>
 #include "util.h"

src/core/display.c

@@ -30,6 +30,8 @@
  * The display is represented as a MetaDisplay struct.
  */
 
+#define _XOPEN_SOURCE 600 /* for gethostname() */
+
 #include <config.h>
 #include "display-private.h"
 #include "util.h"