Tab: track C++ stdlib in use

There are subtle incompatibilities between Apple's libstdc++ and the
libstdc++ used by the various GNU GCC formulae. In addition, we'll
likely also be supporting libc++ in the future, and that's also
incompatible with the other stdlibs.

Tracking it in the tab lets us make sure that dependencies are all
built against the same stdlib to avoid subtle breakage.
Check dependencies for a compatible C++ stdlib

There are now a few possible C++ standard libraries a given build could
be using, with subtle incompatibilities and possibility of breakage
when mixed. This makes sure that the dependency chain was compiled in
a compatible manner.

Fortunately all of the Apple compilers use the same libstdc++, and we
don't yet support building with libc++, so this will primarily only
nag users trying to use GNU gcc who already have software installed
with Apple compilers.

Future TODOs:

* Add general support for building with libc++ (compatibility checking
  already handled here)
* Possibly track formulae which actually build C++ bindings, so that
  users aren't bothered by spurious nagging re: interpreted languages,
  pure-C software, etc.