Add the ability to control the use of AVX extensions via an env var

OCIO already has a set of [Optimization Flags](https://github.com/AcademySoftwareFoundation/OpenColorIO/blob/0c90ded16687b1b63f3e24ef417789c89b1dc509/include/OpenColorIO/OpenColorTypes.h#L567) that may be controlled by an environment variable. It would be nice to have a flag to disable the use of the various SIMD versions of the Lut1D and Lut3D processing code via this mechanism. This would be helpful for both troubleshooting and performance benchmarking.