Experimental support for non-Apple GCCs

This is really preliminary. I didn't change much except a few lines in superenv.

This adds support for building software using a Homebrew-installed, non-Apple GCC. I've tested so far using gcc48 from homebrew/versions, and was able to successfully build gource on Snow Leopard (which can't be built with any Apple-provided compilers on Xcode 3.2.6).

There are a few limitations:

* Only superenv is supported. Not sure if it's worth adding support to stdenv.
* No `--use-foo` switch is supported, only the `HOMEBREW_CC` environment variable. For instance, `HOMEBREW_CC=gcc-4.8`.

To test on < Lion, create a symlink in ENV to enable superenv. (It seems to be working reliably now.)