Allow installation to proceed when CLT is dispersed

@benhoskings commit message should say it all. mind cutting a release with this?
Sorry about the delay :) I've been thinking about this but hadn't come to an answer. Your message pushed it to the front of my mind yesterday.

As far as I can see, the only way out of this is to allow gcc to be installed in a separate location. That's actually no problem I think, because it's legacy on OS X now anyway, and having a separate version of it installed from elsewhere is a thing that's sometimes required.

What do you think?