diff --git a/mps/code/config.h b/mps/code/config.h
index 370542b5a2e..49d14e42e22 100644
--- a/mps/code/config.h
+++ b/mps/code/config.h
@@ -356,7 +356,10 @@
 #if defined(MPS_OS_LI)
 
 #define _XOPEN_SOURCE 500
+
+#ifndef _GNU_SOURCE
 #define _GNU_SOURCE
+#endif
 
 #endif