diff --git a/config.h b/config.h
index f717d00c8a..5cce39ee94 100644
--- a/config.h
+++ b/config.h
@@ -16,7 +16,10 @@
 #  define _POSIX_C_SOURCE 200809L
 #endif
 
-#define _GNU_SOURCE
+
+#ifndef _GNU_SOURCE
+#  define _GNU_SOURCE
+#endif
 
 #define OPENSSL_LOAD_CONF