Reexamine SDL2 build process

[tom-seddon]

Should really use the library names set by find_package: SDL2::SDL2-static or whatever it is.

Looks like there might be some support for doing this even when building SDL2 as part of your project: https://github.com/tom-seddon/SDL-1/blob/e8b6edd5a081d0b45c2ac39ab690e584a6b95a84/CMakeLists.txt#L3391

Ideally this would also tidy up this nonsense as well:

b2/CMakeLists.txt

Line 202 in 4bd1939

# The build process generates an SDL_config.h file that goes in the