build UPDATE use c99 standard instead of gnu99

GNU extensions are not mandatory for a long time now.
diff --git a/tests/test_init_destroy_server.c b/tests/test_init_destroy_server.c
index 64432a7..082710c 100644
--- a/tests/test_init_destroy_server.c
+++ b/tests/test_init_destroy_server.c
@@ -12,6 +12,8 @@
  *     https://opensource.org/licenses/BSD-3-Clause
  */
 
+#define _GNU_SOURCE
+
 #include <errno.h>
 #include <fcntl.h>
 #include <pthread.h>