formula: move cxxstdlib methods to the class.

This allows disabling this checks when e.g. pouring bottles.
gcc 4.8.2 (new formula)
shared: don't warn about non-Apple compilers now.
Support core GCC formula as a GCC compiler.

It is activated by the same mechanism as the Homebrew/versions compilers
which now check if the GCC formula uses the same, correct version.
Use gcc instead of apple-gcc42 when needed.
Migrate apple-gcc42 back to homebrew/versions.