build UPDATE use c99 standard instead of gnu99

GNU extensions are not mandatory for a long time now.
diff --git a/tests/test_thread_messages.c b/tests/test_thread_messages.c
index b894dcc..89eaa02 100644
--- a/tests/test_thread_messages.c
+++ b/tests/test_thread_messages.c
@@ -13,6 +13,8 @@
  *     https://opensource.org/licenses/BSD-3-Clause
  */
 
+#define _GNU_SOURCE
+
 #include <pthread.h>
 #include <stdio.h>
 #include <stdlib.h>