Leverage recent work from the OpenImageIO build system

[doug-walker]

I notice that Larry has recently done a lot of experimentation and development on the OIIO build system. 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?

[lgritz]

Just here to comment that what I did on the OIIO side (which hasn't been merged yet) is very experimental. I'm open to suggestions for how to improve, including telling me that I've overlooked something important, or reinvented the wheel, or not appreciated the wisdom of ExternalProject_Add in some way. I hope it's valuable for you, too. I was very inspired by OCIO's ability to build missing dependencies and statically compile them into the library, it's really helped me over the years. But additionally, I was groping for some way to radically simplify it so that the per-dependency files were as compact and intuitive as possible.