man/examples: set _GNU_SOURCE in source, rather than by compile option

Addresses https://github.com/systemd/systemd/pull/32057#issuecomment-2034408569.

man/event-quick-child.c

@@ -1,5 +1,6 @@
 /* SPDX-License-Identifier: MIT-0 */
 
+#define _GNU_SOURCE 1
 #include <assert.h>
 #include <stdio.h>
 #include <unistd.h>

man/hwdb-usb-device.c

@@ -1,5 +1,6 @@
 /* SPDX-License-Identifier: MIT-0 */
 
+#define _GNU_SOURCE 1
 #include <stdio.h>
 #include <stdint.h>
 #include <systemd/sd-hwdb.h>

man/journal-iterate-poll.c

@@ -1,5 +1,6 @@
 /* SPDX-License-Identifier: MIT-0 */
 
+#define _GNU_SOURCE 1
 #include <poll.h>
 #include <time.h>
 #include <systemd/sd-journal.h>

man/journal-stream-fd.c

@@ -1,5 +1,6 @@
 /* SPDX-License-Identifier: MIT-0 */
 
+#define _GNU_SOURCE 1
 #include <errno.h>
 #include <syslog.h>
 #include <stdio.h>

man/meson.build

@@ -306,10 +306,10 @@ default_args = [
 ]
 
 std_args_in = [
-        [ '-std=c99', '-D_GNU_SOURCE', ],
-        [ '-std=c11', '-D_GNU_SOURCE', ],
-        [ '-std=c17', '-D_GNU_SOURCE', ],
-        [ '-std=c23', '-D_GNU_SOURCE', ],
+        [ '-std=c99', ],
+        [ '-std=c11', ],
+        [ '-std=c17', ],
+        [ '-std=c23', ],
         [ '-std=gnu99', ],
         [ '-std=gnu11', ],
         [ '-std=gnu17', ],

man/notify-selfcontained-example.c

@@ -7,6 +7,7 @@
  * This protocol is guaranteed to be stable as per:
  * https://systemd.io/PORTABILITY_AND_STABILITY/ */
 
+#define _GNU_SOURCE 1
 #include <errno.h>
 #include <inttypes.h>
 #include <signal.h>

man/vtable-example.c

@@ -1,5 +1,6 @@
 /* SPDX-License-Identifier: MIT-0 */
 
+#define _GNU_SOURCE 1
 #include <errno.h>
 #include <stdbool.h>
 #include <stddef.h>