mkmf.rb: use internal path
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@37013 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
mkmf.rb: use internal path
internal to deal with in Makefile.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@37013 b2dd03c8-39d4-4d8f-98ff-823fe69b080e