diff --git a/checkphasta/checkphasta.cpp b/checkphasta/checkphasta.cpp
index 528ffc7f..1b61a5ef 100644
--- a/checkphasta/checkphasta.cpp
+++ b/checkphasta/checkphasta.cpp
@@ -1,4 +1,6 @@
+#ifndef __xlC__
 #define _GNU_SOURCE
+#endif
 #define _WITH_DPRINTF
 #include <cstdio>
 #include <cstdlib>