SDL grouping in CMake

Here are two versions of the grouping patch that I proposed
earlier. When grouping is used in the CMake GUI this forces all the
CMake options into a small number of groups.

The 'SDLOption' set of patches reduces it to two groups. The

‘SDL_’ group has all the SDL subsystems and the ‘SDLOption_’ group has
CPU instructions, video systems, sounds system and various other back
end non-API type of options. The ‘SDL-Only’ lumps all of these
together under one ‘SDL_’ group. I think it is kind of messy but some
people seemed to want this and I think either is an improvement.

Both of these compile on my Linux x64 system with no additional

warnings or errors, in a few different

Please consider this email written permission to do with these

patches as you see fit and re-license as the SDL projects needs.–