common.mk: update only non-existing files

* common.mk (update-config_files): update only non-existing files,
  and get rid of trying to download existing files every time.
  gitweb does not send Last-Modified header.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47742 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

common.mk

@@ -1077,7 +1077,7 @@ up::
 
 update-config_files:
 	$(Q) $(BASERUBY) -C "$(srcdir)/tool" \
-	    ../tool/downloader.rb gnu \
+	    ../tool/downloader.rb -e gnu \
 	    config.guess config.sub
 
 update-gems: PHONY