Make Compiler Dependency Explicit

[Ryan Schmidt]

On Jul 11, 2019, at 18:47, Jahrme Risner wrote:

> As for the compiler blacklist/whitelist/fallback, I was using what was already provided, but if we can make it even more intelligent, that would be wonderful. If I understand you correctly, a Portfile should use either blacklist+fallback OR a whitelist as using the whitelist makes the other two lists largely redundant.
> 
> If that is correct, what would you suggest as the best setup to require a GNU gcc? The current blacklist includes:
> - *clang*
> - *llvm-gcc*
> - *apple-gcc*
> - gcc
> - gcc-3.3
> - gcc-4.0
> - gcc-4.2
> This was crafted to ban all of Apple's fake gccs which are actually clang/llvm in disguise.

I guess it would suffice to use:

compiler.blacklist *clang* gcc *gcc-3.* *gcc-4.*
compiler.fallback-append macports-gcc-[whatever version you want]

though my guidance to avoid setting the whitelist is aimed more at ports that do build using a recent clang but not with older clangs. In this case, I've seen non-ideal constructions used before, in which the intention was to prohibit the use of older clangs and to specify the use of a newer clang, but which, because a specific clang that was new at the time was specified, and because newer clangs were added to the default list in MacPorts base over time, actually resulted in a rather old version of clang being forced instead of the current version which would have worked fine.