merge revision(s) 41648: [Backport #8115]
* 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/branches/ruby_2_0_0@41658 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
merge revision(s) 41648: [Backport #8115]
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_0_0@41658 b2dd03c8-39d4-4d8f-98ff-823fe69b080e