proposal for setting minimum upgraded compiler version

[Greg Troxel <gdt%lexort.com@localhost>]

This is meant to be complementary to the USE_CXX_FEATURES proposal, and
almost entirely separable from it.  It tries to distill some previous
comments.
  
Currently, we can end up with GCC_REQD of various values.  The same will
happen with USE_CXX_FEATURES, via the mapping from features to version.
The same could in theory happen with clang or other compilers.

This results in building multiple gcc versions which

  is wasteful of CPU time, disk writes, space etc

  makes the "different libstdc++" problem worse

  doesn't seem to have any real benefit.

The proposal (which is drafty) is

  define MIN_UPGRADED_GCC_VERSION.   This can be set in platform.mk, and
  if not will be a default in some gcc.mk.  Or it can be set by the
  user.

  Allow defining MIN_UPGRADED_FOO_VERSION for other compilers, even if
  not implemented.

  Whenever base gcc is not good enough, if MIN_UPGRADED_GCC_VERSION is
  >= the version that is required, use MIN_UPGRADED_GCC_VERSION instead.
  Similarly for other compilers, even if not implemented.

  Use MIN_UPGRADED_GCC_VERSION when building gfortran etc.

and that's the entire proposal.  It does not preclude other proposals,
like bootstrap a compiler and use it, or use MIN_UPGRADED_GCC_VERSION
for everything that you don't need to build gcc.  The point is that the
proposal is small with limited consequences and I think we can probably
agree on it and do it, and make progress.

A straw value for MIN_UPGRADED_GCC_VERSION right now is 10, based on my
netbsd-9 box having gcc 7, 8, 9 and 10 all built.  I have no objection
to a higher value, except that it has to build and I don't know about
that.  I have no objection to using different values different places;
picking 12 in general and dropping it to 10 on systems that won't build
12 might be reasonable.  We don't have to agree on the values to agree
on this being an ok approach.