Cactus' ExternalLibraries system removes the "system" locations from any INC_DIRS, but not all compilers look at the "well known" locations on their own. So the machine definition files (in the Einstein Toolkit) try to work around by setting CPATH which is however not used by gfortran, only by cpp.

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Ticket URL: https://bitbucket.org/einsteintoolkit/tickets/issues/2775/flrwsolver-work-around-cactus-include