Remove _GNU_SOURCE macro

After some research, the _GNU_SOURCE does not affect any function call
anymore. To avoid future failures on other systems not using glibc,
_GNU_SOURCE gets removed.

src/dunst.c

@@ -1,6 +1,5 @@
 /* copyright 2012 - 2013 Sascha Kruse and contributors (see LICENSE for licensing information) */
 
-#define _GNU_SOURCE
 #define XLIB_ILLEGAL_ACCESS
 
 #include "dunst.h"

src/markup.c

@@ -1,6 +1,5 @@
 /* copyright 2013 Sascha Kruse and contributors (see LICENSE for licensing information) */
 
-#define _GNU_SOURCE
 #include "markup.h"
 
 #include <assert.h>

src/menu.c

@@ -1,6 +1,5 @@
 /* copyright 2013 Sascha Kruse and contributors (see LICENSE for licensing information) */
 
-#define _GNU_SOURCE
 #include "menu.h"
 
 #include <errno.h>

src/notification.c

@@ -1,6 +1,5 @@
 /* copyright 2013 Sascha Kruse and contributors (see LICENSE for licensing information) */
 
-#define _GNU_SOURCE
 #include "notification.h"
 
 #include <assert.h>

src/option_parser.c

@@ -1,6 +1,5 @@
 /* copyright 2013 Sascha Kruse and contributors (see LICENSE for licensing information) */
 
-#define _GNU_SOURCE
 #include "option_parser.h"
 
 #include <glib.h>

src/utils.c

@@ -1,5 +1,4 @@
 /* copyright 2013 Sascha Kruse and contributors (see LICENSE for licensing information) */
-#define _GNU_SOURCE
 #include "utils.h"
 
 #include <assert.h>