patch 8.2.4039: the xdiff library is linked in even when not used

Problem:    The xdiff library is linked in even when not used.
Solution:   Use configure to decide whether xdiff object files are included.
diff --git a/src/Makefile b/src/Makefile
index aafe139..c65781e 100644
--- a/src/Makefile
+++ b/src/Makefile
@@ -1874,7 +1874,7 @@
 	$(OS_EXTRA_OBJ) \
 	$(NETBEANS_OBJ) \
 	$(CHANNEL_OBJ) \
-	$(XDIFF_OBJS)
+	$(XDIFF_OBJS_USED)
 
 # The files included by tests are not in OBJ_COMMON.
 OBJ_MAIN = \