add in OS X Monotonic clock as well as handling fall-back incase the OSX/Linux system doesn't have a monotonic clock.

Code curtesy of Thomas Habets ( https://github.com/ThomasHabets/monotonic_clock )

--HG--
extra : rebase_source : 767f4fbc2b7bda8d59c7b10a24467043770e94f1
default clock_gettime use to enabled as there is now proper fallback code for systems that don't have it.
move Ticks initialization tracking to separate function and ensure it's called with SDL_VideoInit is called to init SDL instead of SDL_Init

-- why do we even allow initialization w/o calling at least SDL_Init(0) ?