[opus] configure options for x86

[Jean-Marc Valin]

On 11/13/2015 04:40 PM, Sean Darcy wrote:
> --enable-float-approx, enable fast approximations for floating point
> 
> does this have a material effect on encoding speed, or quality ?

The main thing float-approx does is to use polynomial approximations for
log() and exp(). These make the code slightly faster with no impact on
quality. The only reason the option isn't enabled by default is that it
won't work on the (rare) platforms that don't have IEEE 754 floating point.

	Jean-Marc