patch 8.1.0766: various problems when using Vim on VMS

Problem:    Various problems when using Vim on VMS.
Solution:   Various fixes. Define long_long_T. (Zoltan Arpadffy)
diff --git a/src/xdiff/xinclude.h b/src/xdiff/xinclude.h
index 46b8608..752c9f3 100644
--- a/src/xdiff/xinclude.h
+++ b/src/xdiff/xinclude.h
@@ -22,7 +22,11 @@
 
 /* defines HAVE_ATTRIBUTE_UNUSED */
 #ifdef HAVE_CONFIG_H
-# include "../auto/config.h"
+# ifdef VMS
+#  include "[.auto]config.h"
+# else
+#  include "../auto/config.h"
+# endif
 #endif
 
 /* Mark unused function arguments with UNUSED, so that gcc -Wunused-parameter