[cmake][mpi] Use correct guard MPI

The dune CMake configuration actually needs MPI_C_FOUND to be true. If this is true HAVE_MPI is set (see https://gitlab.dune-project.org/core/dune-common/-/blob/master/cmake/modules/DuneMPI.cmake#L33). This variable is suggested to be used as a replacement for MPI_FOUND in this MR.

According to the CMake 3.1 doc MPI_FOUND is actually discouraged and only set for backwards-compatibility with older versions (https://cmake.org/cmake/help/v3.1/module/FindMPI.html). It is reintroduced in newer versions of CMake but is then only true if MPI has been found for all requested languages (whatever "requested languages" means..). Which is also not what we want. Dune seems to only require that MPI_C_FOUND is set (i.e. the MPI C interface is found).