Comment (by Federico Cipolletta):

Ok, I just pushed the doc for the ExternalLibraries/FFTW3 and made a pull request for the Cactus UserGuide with the note that I was talking about. I think that the documentation added is very general and could be applied to all the other external libraries.

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Ticket URL: https://bitbucket.org/einsteintoolkit/tickets/issues/2428/possible-issue-between-externallibraries