mkmf.rb: use expanded values

* lib/mkmf.rb: should use expanded values for header directories
  unless extmk.  patch by vo.x (Vit Ondruch) at [ruby-core:55653]
  [Bug #8115], rhbz#921650.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@41648 b2dd03c8-39d4-4d8f-98ff-823fe69b080e