RUBYLIB spec on Windows
Now that both mswin & mingw are building, some of the previous commits are showing issues.
I added a PR on GitHub at https://github.com/ruby/ruby/pull/1902.
The mswin build & ruby-loco (MinGW) passes with it. Wasn't sure about placement of the first two lines, above makes it very clear, but ENV['RUBYLIB'] certainly is repeated. Could move below and use @rubylib...