Enable MPI even if MPIRUN not in $PATH

When configuring on Cray systems, there is no way to access the mpirun command. It does simply not exist on the login nodes, only on the back-end and not even in a normal form.

At the moment, if we can't run mpirun at the configure time to figure out what MPI version we are running and what level of threading support we have, we simply switch off the support of MPI. Could we avoid that and just trust the user knows what they are doing ? i.e. if MPI has been found but we can't find any form of mpirun, just keep going and leave the variables as they are ?