Comment (by Roland Haas):

Running

bash-3.2$ port diagnose
Error: currently installed version of Xcode, 11.0, is not supported by MacPorts.  For your currently installed system, only the following versions of Xcode are supported:  10.3 10.2.1 10.2 10.1 10.0

indicates that the issue is that my XCode is too new. Unfortunately if I believe the macports ticket, then downgrading XCode will not solve the issue (though maybe one just has to clean it more thoroughly than was tried in the ticket).

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