Import latest xcode.rb from upstream homebrew

[ ] Have you followed the guidelines in our Contributing document?
[ ] Have you checked to ensure there aren't other open Pull Requests for the same change?
[ ] Have you added an explanation of what your changes do and why you'd like us to include them?
[ ] Have you written new tests for your changes? Here's an example.
[ ] Have you successfully run brew style with your changes locally?
[ ] Have you successfully run brew tests with your changes locally?

SuperNEMO-DBD / brew