diff --git a/include/global.h b/include/global.h
index c734ccaf7..5b15963d7 100644
--- a/include/global.h
+++ b/include/global.h
@@ -199,6 +199,7 @@ typedef uchar nhsym;
 #define PORT_ID "Mac"
 #endif
 #ifdef __APPLE__
+#undef SVR4   /* avoid a conflict in X11 header file */
 #define PORT_ID "MacOS"
 #endif
 #ifdef MSDOS