README: get rid of MinGW?

[Artoria2e5]

The old MinGW is very inactive¹ -- even its domain has expired. Its more active fork MinGW-w64 mainly focuses on building toolchains for distributions to package, and on Windows the two would be MSYS2 and Cygwin.

¹ You can't quite call it dead, since the OSDN page is technically kicking with the new GCC releases, and people are actually talking on the mailing list. But they still haven't done 64-bit, which isn't much of a good sign in 2021.

I recommend removing references to MinGW in the README. The references can optionally be replaced with MSYS2 if someone wants to test it -- it's bascially Cygwin but lighter and easier to install packages (via pacman).

[GitMensch]

MSYS2 is still much more heavy than MinGW+MSYS.
And as cccl uses whatever VC setup is active you can perfectly use "simple old MinGW+MSYS" + cccl + VC 64bit.

So I'd vote for a "no" answer and close this issue.

[wsfulton]

Any Unix-like system will work. I've updated the README to reflect this.

cccl and MinGW-w64 and MSYS2 is used by SWIG on Appveyor, so does work.