Fix mingw support with cmake

[madebr]

• add CI github action building librw on msys2 with mingw.
• allow linking with static/shared glfw3/SDL2 + change between them without having to modify paths in the cache.
• statically link to the compiler runtime: on MINGW, this is useful to avoid needing libgcc.dll,libstdc++.dll or libwinpthread.dll.
• add the processor architecture to the generated tar ball filename.
• fix linking on mingw: mark WinMain as undefined, thus not throwing away the symbol from the librw_skeleton library.
• set the default MSVC debugger working directory for each executable to its file location. This was useful for finding the assertion error in Bump imgui to 1.83 #86
• fix linking with SDL2: add SDL_MAIN_HANDLED to avoid renaming main to SDL_main.
• add the ability to install runtime libraries (libgcc.dll,libstdc++.dll or libwinpthread.dll) + glfw3/sdl2 dll library (only when needed).

@ccawley2011 Can you please review this one?

Overrides #83

[madebr]

Ready for review.