diff --git a/src/tini.c b/src/tini.c
index 3ad8232..77f7caa 100644
--- a/src/tini.c
+++ b/src/tini.c
@@ -1,5 +1,7 @@
 /* See LICENSE file for copyright and license details. */
+#ifndef _GNU_SOURCE
 #define _GNU_SOURCE
+#endif
 
 #include <sys/types.h>
 #include <sys/wait.h>