- use find_package(OpenMP) to test that the compiler supports OMP
(looks like the "express" versions of MSVC do not)
- if not available, make sure that osd does not register those
compute kernels (but does register the CPU standalone ones)
- similar refinements on other dependencies (Maya, CUDA) where
the build "opts in" depending on which libs are found.
some CMakeLists still need more cleanup...
Closes#9