- Updated CMake version requirement to avoid a deprecation warning
- Added a check for memory.h. The corresponding header already had a
  #cmakedefine for this, but the check was missing.

- Updated mumps submodule pointer to newest version, which contains
MUMPS 5.4.0
- Needed to make minor changes in the interface due to the fixed fake
MPI implementation in our MUMPS repository, which renames the fake MPI
obects from MPI_* to FPI_*