Fix GNU compiler support

[JayFoxRox]

• First cleans up the project and gets rid of platform specific code.
• Then we switch to C++17; This might break Windows - I'm not sure if VC++ has std::filesystem (although I expect it supports that by now?). If it doesn't, we probably have to use platform specific using fs = ..., or get rid of the affected code. As GCC below version 9 needs a special lib for std::filesystem, I had to add platform specific code again.
• Remaining fixes are done to support non-Windows: case sensitive path was bad + crc32c.cpp used MS intrinsics. I chose to replace it with the upstream xxHash (which is C, not C++, but supports being used as header in C++ explicitly)
• Lastly, I cleaned up the CMake file again

In order to support non-Windows platform, we'll also have to update the submodule for https://github.com/Cxbx-Reloaded/XbSymbolDatabase/pull/54, which is still in review.

Please on Windows to make sure I broke nothing. Ideally also test on macOS and other non-Windows platforms.

I barely did any testing, but the generated ini files seemed to look fine and symbols were detected.

[RadWolfie]

Generate MSVC project files + compile works fine. At last, one xxhash32 warning message is gone. Since it is replaced with better xxhash library.

EDIT: Originally complaining about __int64 and int are not compatible.