build UPDATE use c99 standard instead of gnu99

GNU extensions are not mandatory for a long time now.
diff --git a/src/session_client_tls.c b/src/session_client_tls.c
index 73f8cee..f95fd46 100644
--- a/src/session_client_tls.c
+++ b/src/session_client_tls.c
@@ -16,6 +16,8 @@
  *     https://opensource.org/licenses/BSD-3-Clause
  */
 
+#define _GNU_SOURCE /* pthread_rwlock_t, strdup */
+
 #include <assert.h>
 #include <errno.h>
 #include <string.h>