Add _GNU_SOURCE to all files

If configured with CFLAGS=-std=c99, then quite a few warnings are
raised, as well as a couple of errors. By forcing the system to use
_GNU_SOURCE, we can allow the compilation to succeed even if using a
user defined C standard.

lib/libx52/x52_date_time.c

@@ -6,6 +6,7 @@
  * SPDX-License-Identifier: GPL-2.0-only WITH Classpath-exception-2.0
  */
 
+#define _GNU_SOURCE
 #include "config.h"
 #include <stdio.h>
 #include <stdint.h>

utils/cli/x52_cli.c

@@ -143,6 +143,7 @@ possibly through \b sudo(8)
 
 */
 
+#define _GNU_SOURCE
 #include "config.h"
 #include <stdio.h>
 #include <stdint.h>

utils/test/x52_test.c

@@ -6,11 +6,13 @@
  * SPDX-License-Identifier: GPL-2.0-only WITH Classpath-exception-2.0
  */
 
+#define _GNU_SOURCE
 #include "config.h"
 #include <stdio.h>
 #include <stdlib.h>
 #include <signal.h>
 #include <string.h>
+#include <locale.h>
 #include <time.h>
 
 #include "libx52.h"

utils/test/x52_test_clock.c

@@ -6,6 +6,7 @@
  * SPDX-License-Identifier: GPL-2.0-only WITH Classpath-exception-2.0
  */
 
+#define _GNU_SOURCE
 #include "config.h"
 #include <stdio.h>
 #include <string.h>

utils/test/x52_test_led.c

@@ -6,6 +6,7 @@
  * SPDX-License-Identifier: GPL-2.0-only WITH Classpath-exception-2.0
  */
 
+#define _GNU_SOURCE
 #include "config.h"
 #include <stdio.h>
 #include <string.h>

utils/test/x52_test_mfd.c

@@ -6,6 +6,7 @@
  * SPDX-License-Identifier: GPL-2.0-only WITH Classpath-exception-2.0
  */
 
+#define _GNU_SOURCE
 #include "config.h"
 #include <stdio.h>
 #include <string.h>