Link against libc++ on macOS

[eborisch]

The current (binary blob) macOS release links against libstdc++; since 10.9 (i.e, for five years) the default c++ library from macOS compilers has been libc++.

This trips up a consistency check within MacPorts by being linked against the old library. (And since we can't recompile it ourselves.)

Likewise, versions for 3.6 and 3.7 of the .pyc files would be helpful, too.

[aganders3]

I think this makes sense and I will try to get to it soon (unless @rkrobison wants to do it first!). If we build using the latest version of GPI (1.0.5 as of now) I think the target is 10.9.

This is one of the unfortunate issues surrounding closed source binary distribution of this package.