patch 8.1.0377: xdiff doesn't use the Vim memory allocation functions

Problem:    Xdiff doesn't use the Vim memory allocation functions.
Solution:   Change the xdl_ defines.  Check for out-of-memory.  Rename
            "ignored" to "vim_ignored".
diff --git a/src/globals.h b/src/globals.h
index 3f5b61f..511479d 100644
--- a/src/globals.h
+++ b/src/globals.h
@@ -1621,8 +1621,8 @@
  * can't do anything useful with the value.  Assign to this variable to avoid
  * the warning.
  */
-EXTERN int ignored;
-EXTERN char *ignoredp;
+EXTERN int vim_ignored;
+EXTERN char *vim_ignoredp;
 
 #ifdef FEAT_EVAL
 /* set by alloc_fail(): ID */