Blender current master specifies external library OpenImageIO, version 18.104.22.168 (as of 2021 Feb 4). That OIIO version is about 9 months old, and many versions behind. Please update Blender to use OpenImageIO version 22.214.171.124, or latest.
This is motivated by a probable bug in CMake coding for external dependency handling in older OIIO versions. This resulted in Blender build failures within the MacPorts ecosystem.
There were significant changes to CMake coding in more recent OIIO versions. Therefore I am hopeful but not completely sure that updating the OIIO version will cure this dependency problem. If not, then updating the OIIO version is at least a step in the right direction.
FWIW, here is the link to the downstream bug report, MacPorts #62065. Thanks for your help.