Fix building c47edit using MinGW on Linux

[madebr]

These patches fix building c47edit on Linux using a MinGW toolchain.

[AdrienTD]

Thanks for the PR! It's indeed useful to support other compilers, and also make it compile on Linux (and run with Wine), because you never know how Windows will become these days...

But at the same time I would have to be more careful to not introduce some compile errors that would only trigger GCC/Clang but not VC++ :)

I will review it, there's some things I need to ask.

[Aztec2012]

AppVeyor build failed

[madebr]

error C2338: static_assert failed: 'Unicode support requires compiling with /utf-8'

Since it is happening in libfmt, I suppose this has nothing to do with this pr but with the dependency. I'll apply a patch similar to this one to the vcxproj

[madebr]

With these 2 last commits, the only error remaining for targeting Linux are the windows.h includes (and its use of the win api).

If https://github.com/0danny/re47 ever gets somewhere, it should be easy to port everything to a platform agnostic toolkit, such as SDL.

[madebr]

The commit "Avoid windows-only _wfopen_s" 89e34e1 needs to be reverted, as this breaks the code.

I removed it from the patch set.