mingw: make get_msvcr() a noop + add a CI job testing MSVC Python with GCC

[lazka]

See the commits for details.

Fixes #204

[lazka]

(I initially made the test suite test against multiple compiler types, but that turned out to complicate things a lot, so I abandoned that approach: https://github.com/msys2-contrib/distutils/commit/fa7e35b063318d6909825ba678aedeb672a17219 in case anyone is wondering how that would look like)

[jaraco]

Amended as c9781aef9.

[lazka]

Thanks! And thanks for the improvements.