Comment (by Roland Haas):

This time around compiling gcc9 from source helps. Next will be to find out the order of port install commands to compile only gcc9 (and not everything else) from source. At least until macports fixes the issue in https://trac.macports.org/ticket/58895 .

Specifically I did:

sudo port -N install pkgconfig gcc9 openmpi-gcc9 fftw-3 gsl jpeg zlib hdf5 +fortran +gfortran openssl ld64
sudo port -N uninstall gcc9
sudo port -sN install gcc9

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Ticket URL: https://bitbucket.org/einsteintoolkit/tickets/issues/2290/failure-to-compile-et_mayer-using-gcc-mp-9