Since srb2sdl2 is the default target, CMake should fatal error if it is not able to generate the target (either due to dependencies issues or something else), unless the system has another target available (such as srb2dd). Currently, it simply gives a warning that it will "not be available".

To go above and beyond, perhaps CMake should succeed if and only if at least one target is generated. This may not be necessary if we're planning on making srb2sdl2 the only supported target moving forward.