Bring back LDFLAGS workaround for Ruby 2.1.1

People are reporting that they are still getting errors:

  something wrong with LDFLAGS="-L/Users/<user>/.rbenv/versions/2.1.1/lib "

It was removed in 97b99997 because we
thought Ruby build process doesn't need it anymore. Turns out it does,
and this workaround is effective for the rest of Ruby 2.1.x anyway.