One final reply: You used MacPorts to install GNU Radio ... so why not use it to install all of the IIO stuff? "port search iio" should come back with:
{{{
gr-iio @20190725-9088ac79_1 (science, comms)
Provides augmented functionality for GNU Radio: IIO device source
libad9361-iio @20190910 (science, comms)
libad9361-iio is an IIO AD9361 library for filter design and handling, multi-chip sync, and more
libiio @0.18_1 (science, comms)
libiio is used to interface to the Industrial Input/Output (IIO) Subsystem
libiio-devel @20191104-c9a854f8 (science, comms)
libiio is used to interface to the Industrial Input/Output (IIO) Subsystem
}}}
The GR ports are for GR37 still; we're working on updating everything to be installable for GR37 and GR38 ... and, where possible, GR39-beta. - MLD