This allows disabling this checks when e.g. pouring bottles.

* In certain cases, a C++ software may result in linking to a different
  and incompatible C++ standard library than its dependencies and still
  works fine because it is by design. Examples include GCC, which will
  bootstrap itself and self-host after finish.

* Add a cxxstdlib_check method to formula to request changing the C++
  standard library checking. Currently using "cxxstdlib_check :skip"
  will let a formula skip such checking. This should only be used on
  rare occasions and be very careful.

Closes Homebrew/homebrew#23687.

Otherwise it's possible that certain buildtime dependencies, that won't
be linked against, will block a build.

See Homebrew/homebrew#23115.

A nil stdlib value represents non-C++ code.

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.