Changing default cxx_stdlib to libc++

[Joshua Root]

On 2018-3-9 20:38 , db wrote:
> On 9 Mar 2018, at 01:27, Ryan Schmidt <ryandesign at macports.org> wrote:
>> When would we run this script to delete libstdc++ archives? Presumably, after all legacy users have MacPorts 2.5 and have switched to libc++.
> 
> How about those that build MP from source?

There are no additional problems arising from that, whether you mean
building base from source or building all ports from source. In fact
there's one less problem if you're not downloading any binary archives,
as you can't get ones that are built for the wrong cxx_stdlib.

- Josh