diff --git a/src/config.in b/src/config.in
index 9fbfa15730a..6a476f431eb 100644
--- a/src/config.in
+++ b/src/config.in
@@ -61,6 +61,9 @@ and this notice must be preserved on all copies.  */
 /* Define if we have the X11R6 or newer version of Xt.  */
 #undef HAVE_X11XTR6
 
+/* Define if we have the X11R6 or newer version of Xlib.  */
+#undef HAVE_X11R6
+
 /* Define if netdb.h declares h_errno.  */
 #undef HAVE_H_ERRNO