Re: infer GCC_REQD from USE_LANGUAGES

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

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

GDT> The problem is that setting GCC_REQD based on language leads us down a
GDT> path of using many gcc versions to build things, and that is unsound
GDT> because they do not preserve ABI compat.

No. That was simply my way of assuring that THE ONE GCC chosen will be able 
to deal with whatever C++ level is required.

Let me give some examples ("chosen" GCC means THE ONE SINGLE GLOBALLY 
CAST-IN-STONE NOT-TO-BE-DEVIATED-FROM version chosen ONCE AND FOR ALL 
by infrastructure defaults or user configuration):
(Sorry for shouting, but I feel repeatedly mistaken)

1. Chosen GCC=15. Package sets GCC_REQD=11. Package sets USE_LANGUAGES=c++21
-- Infrastructure decides GCC>=12 needed for C++21, adds GCC_REQD+=12.
=> Need GCC>=12. Build using GCC 15 (*NOT* 12)

2. Chosen GCC=11. Package sets GCC_REQD=11. Package sets USE_LANGUAGES=c++21
-- Infrastructure decides GCC>=12 needed for C++21, adds GCC_REQD+=12.
=> Need GCC>=12. Error out

2. Chosen GCC=12. Package sets GCC_REQD=13. Package sets USE_LANGUAGES=c++21
-- Infrastructure decides GCC>=12 needed for C++21, adds GCC_REQD+=12.
=> Need GCC>=13. Error out

Does that sound reasonable?