Blender Git Loki

Cycles/Optix: Support building the optix kernels on demand.

CMake: `WITH_CYCLES_DEVICE_OPTIX` did not respect `WITH_CYCLES_CUDA_BINARIES` causing the optix kernel to be always build at build time.

Code: `device_optix.cpp` did not count on the optix kernel not existing in the default location.

For this to work, one should have before starting blender

1) working nvcc environment
2) Optix SDK installed and the OPTIX_ROOT_DIR environment variable pointing to it which is not set by default

Differential Revision: https://developer.blender.org/D7400

Reviewed By: Brecht