macports-libcxx: new libc++.dylib for older systems, installed in parallel

[Ken Cunningham]

New software uses new libc++ features that older systems don’t support. 

I put together a proof-of-concept technology preview here for a new libc++.dylib for older systems:

https://github.com/macports/macports-ports/pull/10238 <https://github.com/macports/macports-ports/pull/10238>

it allows std::filesystem, etc to be used on systems < 10.15.

I have a little demonstrator there too that requires these features, mkvtoolnix, updated as an example of how to use it.

No doubt there are warts / a better way forward, but I think this is a reasonable plan to start with, to see how this works out. 

Nothing comes free, however — discussion re: ODR violation possibilities, etc, is here:

https://trac.macports.org/ticket/62426 <https://trac.macports.org/ticket/62426>

Ken
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