Fix broken cmake support for MinGW32

[daschuer]

This fixes building with mingw32 8.1

Unfortunately wx fails with g++ 8.1 due to a wrong cast.
This was fixed here:
wxWidgets/wxWidgets@424f64f27
I can build after using the patch.

[vslavik]

Sorry, I don't understand why does the commit titled "Update CMake support for MinGW32 8.1" do many things, none of which seem to be related to a particular MinGW version?

It's unclear to me why you add a generated config header to the repository when expat doesn't need it to be built on Windows (see: MSVS build projects).

[daschuer]

The issue is that the cmake build in master are completely broken.

I have fist started ein MinGW 6.3, but the shipped header files do not contain all definitions required. The update to the recent MinGW 8.1 fixes the issue.

I did not notice that expat is not required. I just make it work, because it was part of the original cmake files.

When is expat required?

What needs to be done here, to put that int a mergable state?

[vslavik]

The issue is that the cmake build in master are completely broken.

Then that's what the commit message should say...

I did not notice that expat is not required.

It is required. Building it in this crazy-complicated way is not. Again: see the VC++ build, which does not require a generated header you added. Ergo, evidently you don't need it either. Don't add it.

What needs to be done here, to put that int a mergable state?

See above.

[daschuer]

Done.

Are the commit messages OK now?

Did you consider how to proceed with the pending wx build error?

[daschuer]

AppVeyor succeeds now

[vslavik]

Thanks!