Though these are broken and error prone, as they ignore the possibility
of conflicts between SDL, SDL2 and wrappers thereof discarding useful
include path prefix (SDL2/), since SDL and satellite libraries follow
these it's too hard to stay away.
So don't use prefixes for SDL include paths any longer.
- Use standard FIND_PACKAGE_HANDLE_STANDARD_ARGS instead of custom code
- Fix reporting not found dependencies (was broken due to SDL2_ttf vs.
SDL2_TTF), FIND_PACKAGE_HANDLE_STANDARD_ARGS handles this gracefully
