Fix the use of MSVC in an MSYS environment

[Baljak]

Since MSVC can also be used in an MSYS environment (for example the one provided with Git for Windows), we also need to check if the MinGW compiler is used alongside the MSYSTEM environment variable. This expand the MSYS check made in PR #2053

@MonkeyBreaker Could you check that this fixes your issue?
@kanonet Could you check as well that pybind11 + MSYS still work in your cases?

Fix #2079

[MonkeyBreaker]

@Baljak

I confirm that it now work's on my side

Great !

[wjakob]

Shouldn't MSVC appear somewhere in the condition?

[Baljak]

@wjakob I agree, I made the condition like this to allow more compilers than MSVC only.

[kanonet]

I can confirm, that this does work for me with MSYS2+MinGW. I think this is good as you wrote it, this way it does only change behavior for Msys+MinGW. Did someone test MinGW without MSYS?
This is how my original PR #1851 should have looked, to bad it did not. Thx for fixing it!

[Baljak]

Did someone test MinGW without MSYS?

I did and it works. Actually I made PR #2048 because I'm using MinGW alone. Now every configuration should be OK.

[MonkeyBreaker]

Hi !

Any idea when this PR will be merged on master ?
Nothing urgent, as a temporary solution we checkout on branch v2.4.

[wjakob]

Merged!

[wjakob]

Thanks for tracking this down :)

[MonkeyBreaker]

Thank you !