Leverage recent work from the OpenImageIO build system

I notice that Larry has recently done a lot of [experimentation and development on the OIIO build system](https://github.com/AcademySoftwareFoundation/OpenImageIO/pull/4242). The thread in the PR has some good info about various ways of using CMake. I suspect there are probably some good learnings there that could be applied to OCIO's build system.

Some questions for us, when someone with CMake expertise has some time:

1. Special steps were done to handle OCIO as one of the OIIO dependencies, what steps should we be taking to be a better citizen?
2. Should we emulate any of what was done on OIIO to simplify or improve our CMake scripts?