Comment (by Maria Mutz):

Update: got it to compile! The include files being inside the src directory or its subdirectories was causing the problem described. Once I moved the library and everything except the one file listed in the makefile completely outside of Cactus, both this and the undefined reference errors went away.

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Ticket URL: https://bitbucket.org/einsteintoolkit/tickets/issues/2679/how-to-link-an-externally-compiled-library