CI system for PR builds

[Ken Cunningham]

> On Apr 8, 2018, at 03:49, Rainer Müller <raimue at macports.org> wrote:
> 
> We need to stop investing
> so much effort into legacy systems.

True. Making current software build on ancient compliers is a waste of effort. 

Finish the plan to push all those systems to libc++, put forward a modern clang in portconfigure, and then forget about these older systems. 

Users of them who want something to work can put forward patches if they are reasonable.

Someday spin the tickets off to a separate trac somewhere, if there is enough interest.

Legacy systems should not slow down the progress of the current systems. A quick review on repology.org shows many maintainers have only 50 to 60% current ports.

Ken