Error while installing Boost MPI on macOS

Question

I'm trying to install boost mpi from boost 1_78_0 on macOS, but I'm getting this error:

error: Unable to find file or target named
error:     '/mpi//mpi'
error: referred to from project at
error:     'libs/mpi/build'

I've tried various different entries in my project-config.jam:

using mpi ; using mpi : /Users/nick/sfw/openmpi-4.1.2/bin/mpicxx ;

and putting the mpi bin directory in my PATH. I've run ./b2 with debugging output and it looks like my mpicxx is being used, but I'm still getting the error. I've installed this successfully many times on linux using the using mpi directive without a problem.

thanks.