Fix CMake compatibility with older SDL2 config files + improve install tree

[madebr]

• Fixes Error building with macOS Monterey and CMake 3.24 #77
• Installs CMake configuration files such that people can use SDL2_sound in their CMake project as follows:

  find_package(SDL2_sound REQUIRED CONFIG)
  target_link_libraries(mygame PRIVATE SDL2_sound::SDL2_sound)

  This is also tested on CI

A Linux install tree looks like follows:

prefix
├── bin
│   └── playsound
├── include
│   └── SDL2
│       └── SDL_sound.h
└── lib64
    ├── cmake
    │   └── SDL2_sound
    │       ├── SDL2_soundConfig.cmake
    │       ├── SDL2_soundConfigVersion.cmake
    │       ├── SDL2_sound-shared-targets.cmake
    │       ├── SDL2_sound-shared-targets-debug.cmake
    │       ├── SDL2_sound-static-targets.cmake
    │       └── SDL2_sound-static-targets-debug.cmake
    ├── libSDL2_sound.a
    ├── libSDL2_sound.so -> libSDL2_sound.so.2
    ├── libSDL2_sound.so.2 -> libSDL2_sound.so.2.0.1
    ├── libSDL2_sound.so.2.0.1
    └── pkgconfig
        └── SDL2_sound.pc

A MinGW install tree looks like follows:

prefix
├── bin
│   ├── playsound.exe
│   └── SDL2_sound.dll
├── include
│   └── SDL2
│       └── SDL_sound.h
└── lib
    ├── cmake
    │   └── SDL2_sound
    │       ├── SDL2_soundConfig.cmake
    │       ├── SDL2_soundConfigVersion.cmake
    │       ├── SDL2_sound-shared-targets.cmake
    │       ├── SDL2_sound-shared-targets-release.cmake
    │       ├── SDL2_sound-static-targets.cmake
    │       └── SDL2_sound-static-targets-release.cmake
    ├── libSDL2_sound.a
    ├── libSDL2_sound.dll.a
    └── pkgconfig
        └── SDL2_sound.pc

[ericoporto]

Hey, this is pretty good! These config CMake files are very useful and would be really cool to have a release tag including this PR so they can propagate to the package managers.

[sezero]

Is the patch finalized?

[madebr]

Is the patch finalized?

I think so. Let it settle for a day for eventual passers-by, and merge.

[sezero]

This is in now.