Added SDL2 CMakeLists.txt

[odrevet]

The SDL2 example can be build with this CMake configuration file

[baylej]

Hi @odrevet, thank you for your contribution.

My thoughts:

• You should use find_program to locate sdl2-config.
• Your reference to SDL2_image is constant and hence not portable.
• As sdl2-config does not provide us with the location of SDL_image, we need a FindSDL_image.cmake script.

Most SDL2 projects using cmake have their own FindSDL2.cmake script, but this will no longer be needed since bug 2464 has been fixed.

Every SDL plugins (SDL2_image included) still do not provide downstream projects with exported cmake targets (they don't even have a CMakeLists.txt). This is more problematic...