Compiles now on older Linux distributions too.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@9573 a5019735-40e9-0310-863c-91ae7b9d1cf9
diff --git a/drd/tests/pth_barrier_race.c b/drd/tests/pth_barrier_race.c
index 8aee90e..2255a0e 100644
--- a/drd/tests/pth_barrier_race.c
+++ b/drd/tests/pth_barrier_race.c
@@ -10,6 +10,8 @@
  */
 
 
+#define _GNU_SOURCE
+
 #include <pthread.h>
 #include <stdlib.h>
 #include <unistd.h>