[lmi] Availability of msw gcc binaries

[Greg Chicares]

This would seem to be the canonical MinGW-w64 download page:
  https://downloads.sourceforge.net/mingw-w64
but the latest version there is gcc-8.1.0, which is pretty
old: gcc.gnu.org released 8.1 on 2020-03-04, and they're
already up to 10.2 .

(There is some sort of 9.x release on that site (though if I
paste this URL back into my browser, it doesn't work):

https://sf.net/projects/mingw-w64/files/Multilib%20Toolchains%28Targetting%20Win32%20and%20Win64%29/ray_linn/gcc-9.x-with-ada/

...but it has only fifty-nine total downloads; I get the
impression that it's just a courtesy directory for someone
who wanted to upload his own builds, which may not have
been vetted by anyone else; and if his goal was to make
ada work, it might not be suitable for us.)

Meanwhile, debian 'testing' has MinGW-w64 version 10.1:
  https://packages.debian.org/bullseye/g++-mingw-w64

Vadim--Do you know any reason why the "official" native
binaries would be two years behind debian's cross builds?
Could it be that almost everyone who compiles with gcc on
an msw box is already using 'wsl'?

At any rate, this is just one more reason for migrating
all our development to linux.