Don't use fast math

[illwieckz]

See:

• https://github.com/Unity-Technologies/crunch/pull/16 by @blaztinn

He said:

Without fast math we get the same output for the same input on all the platforms and architectures.

Otherwise the shasum of output was different on different machines that compressed the same input.

So we may want to disable fast-math, and maybe even pass -nofast-math to be 100% sure some environment variables doesn't introduce it again. The related patch only affects the legacy Makefile so someone has to implement it in CMake as well.