Add supported compilers MinGW 8, 9, 10

[ghost]

Add supported compilers MinGW 8, 9, 10 (x86_64 and i686) on Windows 10.

[milancurcic]

Thanks for testing, change looks good to me.

[awvwgk]

@brocolis Did you install the GCC versions from http://winlibs.com/? We might be able to download the corresponding GCC versions from https://github.com/brechtsanders/winlibs_mingw/releases/ and actually test them in the CI to keep supporting a wide range of compilers on Windows as well.

[ghost]

@awvwgk I did download and build MinGW/GFortran from sources. I used the scripts written by https://github.com/StephanTLavavej/mingw-distro/ with minor modifications.

[awvwgk]

Thanks for the clarification. Using this approach would be taxing on the CI runners, if you find a good way to (semi-)automatically install different GCC versions on Windows, let me know, I would be interested to extend the stdlib CI testing for those at some point.

[ghost]

Hello. I'm not familiar with CI setup. Anyway, I'll upload the binaries/build scripts to the internet in the next few days.

[ghost]

Hello. I'm not familiar with CI setup. Anyway, I'll upload the binaries/build scripts to the internet in the next few days.

OK, mingw/gfortran-8.4.0, 9.3.0 and 10.2.0 for Windows (i686, x64) are now available at https://fortran.rocks/all.html

[jvdp1]

Sorry for the delay.
I approve this change, as it was suggested.