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.
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