Treat HOMEBREW_CC the same as --cc

With the addition of the `gcc` formula, I would find it very useful to be able to set the default compiler to the Homebrewed GCC compiler using `HOMEBREW_CC`.

+ The current behaviour that `COMPILER_ALIASES` is only applied to `HOMEBREW_CC` and not to `--cc` is maintained.
+ Setting `HOMEBREW_CC` to an invalid value now produces an error message, which I feel is preferable to the current behaviour of silently ignoring the configuration and defaulting to `MacOS.default_compiler`.