[SDL] Do not attempt to use versions < 2.0.0

Versions prior to 2.0.0 are not ABI-compatible with 2.0.x and
attempting to use those will result in random instability. We now
explicitly check the SDL2 version before enabling the SDL2 backend.
[SDL] Removed unnecessary if-branch
Merge pull request #126 from thefiddler/sdlversion

[SDL] Do not attempt to use versions < 2.0.0