Compiler name baked into our packages

[Jeremy Lavergne]

> But if we restrict MacPorts to fetch only packages built of the same compiler and/or Xcode version, then we'd need more buildbots to build those extra packages.

We don't really need more than one buildbot per OS: we simply restrict what we build to the most current free version of Xcode for each OS.

This will also help us rebuild the buildbot packages immediately upon an Xcode update.