diff --git a/src/posix_poll.c b/src/posix_poll.c
index a5016a13..0628cd0c 100644
--- a/src/posix_poll.c
+++ b/src/posix_poll.c
@@ -26,7 +26,9 @@
 // It is fine to use C99 in this file because it will not be built with VS
 //========================================================================
 
+#ifndef _GNU_SOURCE
 #define _GNU_SOURCE
+#endif
 
 #include "internal.h"