[BUILD] NanoVDB - OptiX Root Directory

[nachovizzo]

Environment

Operating System: Ubuntu 20.04 Version / Commit SHA: feature/nanovdb CMake Version: 3.18 Compiler: GCC 9

Describe the problem

Can't find the OptiX root directory

$ cmake ..
 /// etc ...
CMake Warning at cmd/CMakeLists.txt:235 (message):
   - OptiX not found for Viewer. Removing OptiX support.

I've checked the FindOptix.cmake that's in the repo and I found that the variable ${Optix_Root} is being used but not exposed to the user through cmake: https://github.com/AcademySoftwareFoundation/openvdb/blob/9b79bb0dd66a442149083c8093deefcb03f881c3/nanovdb/cmake/FindOptiX.cmake#L26

If it helps it's quite easy to fix in Linux, just run this and it works

$ cmake -DOptiX_ROOT=<PATH_TO_OptiX> ..

I know this feature branch is still a work in progress, so I'm not complaining :angel: I'm just raising the issue in case any other developer is seeing the same issue.

I'm happy to expose this flag to the build system if you are accepting PR's

[kmuseth]

looking into this now