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: mark PHONY targets

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47743 b2dd03c8-39d4-4d8f-98ff-823fe69b080e