mkconfig.rb: use configured libdir value

* tool/mkconfig.rb: use configured libdir value to fix
  --enable-load-relative on systems where libdir is not default value,
  overridden in config.site files.  [ruby-core:47267] [Bug #6903]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@38776 b2dd03c8-39d4-4d8f-98ff-823fe69b080e