Mac: Using ccache or distributed compilation or any other different compiler got much more difficult

Since the mac mkspecs now resolve all tools to a Xcode path via xcrun, instead of taking the tools from the PATH, it's much more difficult to use any kind of distributed compilation system or ccache for faster builds.

What I've been doing so far for Qt 5.1 is to set the CC and CXX make variable to the actual binaries to use when running make.

It would be good to allow overriding that behavior, or find a better way to do it (and document it)