build UPDATE use c99 standard instead of gnu99

GNU extensions are not mandatory for a long time now.
diff --git a/src/messages_client.c b/src/messages_client.c
index f655b5b..04c0b3c 100644
--- a/src/messages_client.c
+++ b/src/messages_client.c
@@ -13,6 +13,8 @@
  *     https://opensource.org/licenses/BSD-3-Clause
  */
 
+#define _GNU_SOURCE /* pthread_rwlock_t, strdup */
+
 #include <assert.h>
 #include <ctype.h>
 #include <stdarg.h>