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.