diff --git a/include/chibi/features.h b/include/chibi/features.h
index 8c933bcc..fdb6fe98 100644
--- a/include/chibi/features.h
+++ b/include/chibi/features.h
@@ -197,8 +197,10 @@
 #define SEXP_BSD 1
 #else
 #define SEXP_BSD 0
+#ifndef _GNU_SOURCE
 #define _GNU_SOURCE
 #endif
+#endif
 
 #ifndef SEXP_USE_NO_FEATURES
 #define SEXP_USE_NO_FEATURES 0