Enable MPI if path is given

Assume that ./configure --with-mpi= also implies --enable-mpi; if you absolutely want to specify path without enabling (?!) then you can do --with-mpi=... --disable-mpi (in that order).

This patch look for anything after the equal sign of the --with parameter and only set the variable `_MPI_PREFIX_PATH` if that is given; it always toggle `USE_MPI`.

(No, this is not a suggested fix for #281; but I discovered this during some testing for that)