C++20 and gcc12/10

[David Holland <dholland-pkgtech%netbsd.org@localhost>]

Recently USE_CXX_FEATURES=c++20 started demanding gcc12. This forces
netbsd 10 to build gcc12, and then to rebuild almost everything with
it (because multiple libstdc++ versions at once doesn't work) and this
is a pretty heavy tax.

Given that gcc10 says that its c++20 support is inadequate, it's
reasonable for the setting to have that effect, I guess.

However, lots of things marked c++20 build just fine with the gcc10 in
-10, including fairly widely used stuff like poppler.

Is it reasonable to commit

  -USE_CXX_FEATURES=      c++20
  +# gcc10's C++20 support seems to be adequate and since an explicit
  +# requirement of C++20 requires building gcc12 on 10.x and elsewhere,
  +# hold off on the stricter constraint for now.
  +#USE_CXX_FEATURES=     c++20
  +GCC_REQD+=             10

in packages where it works? It's easy enough to put back the stronger
constraint if the build stops working.

(Or would it be better to add a new CXX_FEATURE for this, like
c++20-gcc10 or something? I am not at all clear on how any of this
interacts with clang.)

-- 
David A. Holland
dholland%netbsd.org@localhost