diff --git a/Documentation/ptp/testptp.c b/Documentation/ptp/testptp.c
index f1ac2dae999e..ba1d50200c46 100644
--- a/Documentation/ptp/testptp.c
+++ b/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)
 {