Only sign-expand the nibble when needed
Only require MSADPCM block alignment to be a multiple of 2
Improve int-to-float and uint-to-float conversions
Use the correct array size
Update config file comments
Remove the Apple-specific atomic function implementations.

I'm not sure if they're even used, but they were rather ugly and are set to get
even uglier since they don't follow normal conventions (missing exchange, and
cas not returning the original value).
Return the original value from CompExchange*