cxx11 portgroup: C++11, C++14, C++17

[Ken Cunningham]

> On Feb 4, 2018, at 9:36 PM, Ryan Schmidt <ryandesign at macports.org> wrote:
> 

> We should think about how this will work if and when it is integrated into MacPorts base and not a portgroup. I think starting with configure.cc_std and configure.cxx_std might work well.
> 


I’m trying to really understand your approach here, which I recognize to be to use the oldest compiler that can do the job.

In my mind, I’d use the newest compiler that the systems can run. 

I know you don’t agree with that, but:

1. all consistent across the systems
2. less testing, less tickets, less crapola to deal with
3. newer optimizations, better speed, etc
4. only port builders need to install the compiler - and they likely want it anyway. Users installing the ports don’t have to, they just download the built products off the buildbots.
5. fewer requests for upstream to hold back to old compiler standards they don’t want to support anyway...

So long as you can bootstrap to the new compiler, why not go with that?

So with this in mind, I’d flush everyone who is not running a current system (past three say) to clang-5.0, or 6.0 if that’s stable enough now.

Easy, quick, done…

Ken