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/config.mk.in b/src/config.mk.in
index df09cfe..4eb6928 100644
--- a/src/config.mk.in
+++ b/src/config.mk.in
@@ -37,6 +37,8 @@
 X_EXTRA_LIBS	= @X_EXTRA_LIBS@
 X_LIBS		= @X_LIB@
 
+XDIFF_OBJS_USED	= @XDIFF_OBJS_USED@
+
 LUA_LIBS	= @LUA_LIBS@
 LUA_SRC		= @LUA_SRC@
 LUA_OBJ		= @LUA_OBJ@