For most formulae, the bottles need a minimum libstdc++ rather than a
minimum GCC version. This is particularly important when building on
Ubuntu where the default compiler version is older than libstdc++.
So, checking the host libstdc++ version is a more accurate way to
determine whether brew GCC is needed at runtime. This can be improved in
the future to check symbol versions (e.g. GLIBCXX, CXXABI, GLIBC) which
can allow some bottles to be installed even with older glibc/libstdc++.
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