If I create a thornlist containing just
the configuration will compile but it won't link. The linker errors indicate that the "Cactus" virtual thorn is using MPI symbols which are not available. The mpi library is not on the linker command line, nor in LIBS. LIBS is set from MPI_LIBS in make.link, but MPI_LIBS is empty. MPI_LIBS is picked up from the MPI configure.sh script, and it is inserted into bindings/Configuration/Capabilities/make.MPI.defn. This is picked up by bindings/Configuration/Thorns/make.Cactus.defn, but I don't think that this file is used by anything. If I create a new empty thorn which uses the MPI capability, the executable is linked successfully. I think that nothing is loading the make.Cactus.defn definitions for the Cactus virtual thorn.
This subtle bug might have other effects so it should probably be fixed.