extmk.rb: untouch makefiles

* ext/extmk.rb (extmake): leave makefiles untouched if the content is
  not changed, to get rid of unnecessary re-linking.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40973 b2dd03c8-39d4-4d8f-98ff-823fe69b080e