* tool/mkconfig.rb: remove prefix from rubyarchdir.
  r39267 expands variables, it changes expansion timing,
  breaks RbConfig::CONFIG["includedir"] and building
  extension libraries with installed ruby.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39284 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* 2013-02-17

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39285 b2dd03c8-39d4-4d8f-98ff-823fe69b080e