Feedback on clang change (#53194)

[Marcus Calhoun-Lopez]

In the recent mailing list discussion on C++11, there were no comments on the specific aspects of the proposal
(https://lists.macports.org/pipermail/macports-dev/2017-January/thread.html#35173).
There is one aspect in particular that I would like to ask about: the proposed changes to clang (https://trac.macports.org/ticket/53194).
Any comments would be greatly appreciated.

There are three ways to allow clang to refer to MacPorts libstdc++.

1) Create a *default* variant to have -stdlib=libstdc++ refer to MacPorts libstdc++.
    This was quite rightly rejected because clang should be “as compatible with the host as possible.”
    I only bring it up because the required patch is simpler.

2) Create a new command switch -stdlib=macports-libstdc++ to refer to MacPorts libstdc++.

3) Create a new subport clang-libstdcxx for which -stdlib=libstdc++ refers to MacPorts libstdc++.
    Again, a reason to do this is because the required patch is simpler.

#2 is already in the development versions of clang
(https://github.com/macports/macports-ports/commit/c68d69b898fece7fea96b61ae0dd33650f628289
and
https://github.com/macports/macports-ports/commit/49bd2ee7c1e54f290cb6fe07a084de8a2abddb66),
but for my purposes, the variant would have to be the default.

Patches for all three proposal are attached to the ticket.

Thank you,
Marcus