mkmf.rb: use configured libdir value
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@37153 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
mkmf.rb: use configured libdir value
default library path. [ruby-core:43726] [Bug #6207]
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@37153 b2dd03c8-39d4-4d8f-98ff-823fe69b080e