Comment (by Roland Haas):

Such an option exists as of git hash 85946a3c "sim-build: fix --jobs case where MAKEJOBS is specified in ini file" of simfactory2 actually using almost exactly the logic Ian suggested (but “invented” independently though I just made simfactory accept a --jobs or -j option to set MAKEJOBS and take a default from the machine database).

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Ticket URL: https://bitbucket.org/einsteintoolkit/tickets/issues/1198/add-an-option-to-sim-build-to-disable