Switching cxx_stdlib for C++11 on legacy macOS (was: Re: [GSoC] migration)

[Ken Cunningham]

> 
> As stupid as it seems but I don't see any cxx_stdlib option in my macports.conf (see attached).

base defaults it to stdlibc++ on <10.9, and libc++ on 10.9+ behind the scenes. 

> 
> Sure, this is a helpful point to keep in mind. Though, any reasons to not switch to libc++?

Although I personally would have preferred if macports switched wholesale to libc++ for all Intel systems for consistency, Marcus' additions to clang-3.9+ allowing clang to link nicely against libgcc does the trick quite well in the end, and actually offer some benefits in cases (gcc6 offers thread-local-storage on systems that libc++ does not).

There are some hassles with this due to idiosyncracies between the headers <https://github.com/macports/macports-ports/pull/717> that we would not have otherwise had to deal with, but these are minor enough I guess.


Ken