Enable OSL now built with LLVM 16.
After OSL is updated to 1.13.6.1 it can be built with clang 16. OSL's use of clang 15 created issues such as #2 (closed) which caused it to be disabled in blender. blender requires a back-port to be compatible with OSL 1.13.5. The back-port will be part of blender 4.1.
Marking as a draft pending the OSL 1.13.6.1 update.