I wonder if this can/should be addressed in the cmake files directly instead of the readme

I disagree, I think this is the better solution. My research so far on getting CMake to find the PyEnv path involves changing things that would probably break FindPython for the normal path, so we'd still need to have some kind of property the user would need to provide to signal to cmake that it needs to override where FindPython looks. And even then it would only be for PyEnv, if there are any other nonstandard ways to install Python we would also need to add overrides for those as well. Compared to that, asking users of non-standard python installation to provide the path to their interpreter this way seems very reasonable to me.

Well, as soon as I finished typing that I found this which might do what we need: pyenv/pyenv#310 (comment)

i'm not opposed to just updating BUILDING.md, just wanted to make sure we weighed our options.

thanks for looking into this @leggettc18!