Consider modern mingw-w64 UCRT build

[matthew-brett]

Building Scipy with Meson, I found I needed to rebuild OpenBLAS with mingw-w64, using the new UCRT linkage.

https://github.com/matthew-brett/openblas-libs/blob/ucrt-build/build_openblas.ps1

In particular, I got the compiler from the rtools, installed with:

choco install rtools

but that is, I believe, equivalent to the latest mingw-w64 UCRT compiler chain.

Would it be reasonable to use that toolchain for the standard CI compile here?

[mattip]

We have moved our windows builds to visual 2017 and are on the cusp of moving to 2019, so as I understand UCRT would be fine.

[matthew-brett]

I think I've cracked this one, with #82

[mattip]

Closing, #82 was merged