MinGW cross compilation support

[D4N]

Required steps for MinGW cross compilation support:

• Fix the windows build in Mingw cross master #645 and remove the binary files from that PR
• Add a MinGW build to the GitLab CI pipeline. Fedora's MinGW can already build exiv2 as a static library, dynamic linking is still broken though. Todo @D4N
• Remove the templating of BasicError and drop WError, instead convert the wchar_t string to a char_t string in the few cases where this class is actually used. Then we can drop the awkward function BasicError<charT>::setMsg() which is exported from a template function. This hopefully fixes dynamic linking on MinGW. See: [WIP] Fix the Error type #685.
• Run the test suite for MinGW with wine on Linux. Notes:
• Wine requires X to be running, so we'll probably have to use something like Xvfb.
• set export WINEDEBUG=-all otherwise wine logs to stderr
• The MinGW dlls are by default not known by wine, they have to be added to WINEPATH. On Fedora this results in: export WINEPATH=/usr/x86_64-w64-mingw32/sys-root/mingw/bin/
• Wine seems to change the line ending, at least the python test complain about whitespace changes at the end of all lines. This can be probably fixed by a simple search & replace when running under wine.

[clanmills]

Thanks for getting involved in this, Dan. I thought I had already removed the binary DLLs. Perhaps not.

I would be really great to see the end of that BasicError::setMsg() puzzle.

[clanmills]

I think this has been dealt with elsewhere and can be closed. The CI performs a cross platform build.