infer GCC_REQD from USE_LANGUAGES (was: nghttp2 broken)

[Edgar Fuß <ef%math.uni-bonn.de@localhost>]

> Seems to me that pkgsrc should know what GCC version is required for a
> declared language (so c++14 should imply a minimum GCC_REQD).
Yes.

I proposed a patch for that, but after endless discussions, people couldn't
agree on the details.

In particular, joerg@ repeatedly turned down proposed solutions with his
I-know-better-attitude -- he may in fact really know better, but failed to 
explain what the particular problem with a proposed solution was.

There were multiple attempts to get this sorted out correctly, but each and 
every one was discussed to death with people resorting to works-for-me-type 
workarounds.

See https://wiki.netbsd.org/pkgsrc/gcc/ for gdt@'s heroic attempts to summarize 
the state of affairs and various discussion threads.