Changing default cxx_stdlib to libc++

[Ken Cunningham]

> On Mar 9, 2018, at 06:18, Joshua Root <jmr at macports.org> wrote:
> 
> We may have to create some bootstrap versions of toolchain ports in
> order to achieve this, so that they can be automatically installed as
> dependencies with no manual intervention.

Maybe we could just force clang 3.4 to build against libstdc++. -- perhaps dependant on whether libcxx is installed or not.... 

Ken