The version check is required for supporting macOS PPC with SDL 2.0.1
and Travis-CI (Ubuntu Trusty) with SDL 2.0.2.
The client now requires SDL 2.0.5 runtime if compiled against SDL 2.0.5
or newer.
This is a little bit of future-proofing, but also gives us a little more
flexibility in general; now we can add in the cvars to open a specific
device, etc, that the OpenAL codepath does.
In SDL2, the initialized subsystems are referenced counted, so it's safe to
initialize them twice, and it makes the SDL_QuitSubSystem during our shutdown
correctly decrement the count. Before (as a probably-harmless bug), it would
not increment the refcount if the subsystem was already initialized, causing
problems when it decremented it later.
