I think I might have made some headway... anyway, I'll post my discoveries here in hopes of providing some other poor soul who might be in my place with some clues on how to proceed.

First of all: that environment variable question in Linux: basically, you do:

export [ENVIRONMENT_VARIABLE]=~/path/to/what/you/want/cmake/to/see

In my case, there were two env vars:

IMAGEIO_LIBRARY_PATH
and
THIRD_PARTY_TOOLS_HOME

So I did this:

export IMAGEIO_LIBRARY_PATH=~/oiio/build/linux

and

export THIRD_PARTY_TOOLS_HOME=~/blender-cycles/boost

I did this because it was suggested in the README, the INSTALL and here:
https://sites.google.com/site/openimageio/checking-out-and-building-openimageio
that this be done.

The error count only changed slightly when I tried to:

make j4 install

What did fix it was going into cmake-gui, discovering where all the holes (missing file references) were, and navigating to those with the gui.
Then, it compiled: mind you, not without issues, but it compiled.

Proof is in the pudding... now, I need to sort out how to build Blender using not the files that came with the svn (would you believe we're still downloading Boost 1.34!!!... Boost is on version 1.51) but with oiio and boost stuff I just built.

:blink:

Wish me luck! I'll keep you posted.

Monterey/Mint21.x/Win10 - Blender3.x - PP11.3(cm) - Musescore3.6.2

Wir sind gewohnt, daß die Menschen verhöhnen was sie nicht verstehen
[it is clear that humans have contempt for that which they do not understand] 

Metaphor of Chooks