Comment (by Roland Haas):

The ticket got sidetracked a bit. The original question was to remove CPPFLAGS and CXXFLAGS from the link call since there is no actual compilation happening (it really only links library and object files).
Removing CXXFLAGS may result in (fixable) build failures in option lists that set eg -fopenmp only in CXXFLAGS and not also in LDFLAGS (or OPENMP_CXXFLAGS and not also OPENMP_LDFLAGS) since -fopenmp must be present in the link line to link in the OpenMP runtime libraries.

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Ticket URL: https://bitbucket.org/einsteintoolkit/tickets/issues/2553/cactus-link-command-uses-cppflags-and