Simplify the CMake scripts, fix warnings and add ci

[madebr]

We (=SDL maintainers) received a pr today adding stsound support to SDL_mixer in libsdl-org/SDL_mixer#650

When looking at the StSound CMake scripts, we found it was very specific to Visual Studio 2017.
CMake is supposed to be usable for almost all targets today, so I made it readily usable on all platforms.

• The first commit simplifies the CMake script, and also adds support for installation and testing
• The second commit fixes a cast warning I noticed when building with mingw64
• The third commit hides the MSVC-specific #pragma comment(lib,"winmm.lib") from non-msvc toolchains
• The fourth commit fixes -Wunused-parameter warnings
• The fifth commit adds GitHub ci support. The result looks like this. MSVC currently fails because of precision loss and/or signed/unsigned mismatches. Those can probably be fixed by using explicit casts. But I did not fix those because I felt it was better for you to have a look at these.