ptp: In the testptp utility, use clock_adjtime from glibc when available

clock_adjtime was included in glibc 2.14. _GNU_SOURCE must be defined
to make it available.
Signed-off-by: Christian Riesch <christian.riesch@omicron.at>
Cc: Richard Cochran <richardcochran@gmail.com>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/ptp/testptp.c

@@ -17,6 +17,7 @@
  *  along with this program; if not, write to the Free Software
  *  Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
  */
+#define _GNU_SOURCE
 #include <errno.h>
 #include <fcntl.h>
 #include <inttypes.h>
@@ -46,12 +47,14 @@
 #define CLOCK_INVALID -1
 #endif
 
-/* When glibc offers the syscall, this will go away. */
+/* clock_adjtime is not available in GLIBC < 2.14 */
+#if !__GLIBC_PREREQ(2, 14)
 #include <sys/syscall.h>
 static int clock_adjtime(clockid_t id, struct timex *tx)
 {
 	return syscall(__NR_clock_adjtime, id, tx);
 }
+#endif
 
 static clockid_t get_clockid(int fd)
 {