Comment (by Roland Haas):

note that this issue will never show up with gcc, since it always has a file omp.h in the include search path, but Apple's clang on macOS, which does not support OpenMP at all, does not provide such a file, hence the error shows up.

--
Ticket URL: https://bitbucket.org/einsteintoolkit/tickets/issues/2946/carpetx-includes-when-openmp-is-not