Added SRB2_CONFIG_USE_INTERNAL_LIBRARIES option (defaults to off right now),...

Added SRB2_CONFIG_USE_INTERNAL_LIBRARIES option (defaults to off right now), some WIP work on making it ...actually do what it says on the tin.

Basically, it manually sets *_FOUND, *_INCLUDE_DIRS and *_LIBRARIES instead of using find_package. Frankly I have no idea how well what I've done works currently though, not even sure if I've set the _LIBRARIES variables correctly. Again, it's WIP work, this can probably be fixed eventually I suppose.