Add GCC formula to core, use it.

Apple GCC 4.2 proved useful for a while but it's not really meeting our needs. Many people seem to want to install `gcc`, some packages still require it (and may forever) and Apple GCC 4.2 doesn't support any C++11 stuff.

Instead, let's pull in GCC as a new formula to core, add the necessary plumbing for it to be detected, set it to a priority just under Clang and adjust the various dependencies and messaging accordingly.

Please give this some testing if you get the chance too. Ta!