We currently always default to building with GCC on OS X. However, at some point recently Apple have changed the default compiler ('cc' and 'c++') to be clang, rather than GCC. Our build system does not honor this default, but it should.

We need to figure out if the change in the default aligns with the OS revision, or with the XCode revision, but in any event we should mirror it, so that on modern systems the default compiler for Apple is clang.