C23 compiler selection

tl;dr

  At wiz's prompting, I have just committed compiler selection for C23.

  Previously, adding USE_CC_FEATURES=c23 did nothing at all.

  Now it forces gcc >=14, aligned to NetBSD current.

details;


I first updated comments a lot, but not in any radical way

I didn't start a discussion because there was only one choice to make:
should c23 lead to gcc 14, or 15?  As is increasingly usual, the
language is complicated and there are a few (probably obscure) features
that need 15, while almost everything works on 14.

I guessed that 14 would be sufficient for the packages we will have that
need c23 (because no upstream should be requiring it yet anyway, there
likely won't be that many, and they probably won't use the most obscure
features), and that allowing 14, and thus avoiding a compiler build on
at least NetBSD current and perhaps others, would be beneficial.

We can certainly revisit this, if packages declaring c23 fail with 14
and work with 15, and when/if current imports 15 or 16.