Make Compiler Dependency Explicit

[Ryan Schmidt]

On Jul 11, 2019, at 19:42, Richard L. Hamilton wrote:

> I've hit some ports yesterday on Snow Leopard (only have one that old) that require clang-3.9...but clang-3.9 is deemed superseded by clang-8.0, except the ones with clang-3.9 dependencies won't use that, so it's a no-win upgrading those. The problematic ports:
> py27-cython
> xorg-libXevie
> xorg-libXfontcache
> xorg-libXp
> xorg-xorgproto

At the same time that Jeremy removed clang-3.9 and clang-4.0 and marked them replaced by clang-8.0, he also went through all the ports and removed all the references to clang-3.9 and clang-4.0 that he could find. I recently found and removed a few more, and filed a couple tickets about others I couldn't fix right then. If you've found other remaining references to removed compilers, please file tickets about those ports so that we can fix them.