Boost: enforce '--with-mpi' with '--without-single'

* Building MPI support for both single and multi-threaded flavors will
  result in duplicated rules for mpi.so.  Upstream bug report
  https://svn.boost.org/trac/boost/ticket/8841.
* Require '--with-mpi' be used together with '--without-single' until
  this is fixed.

Closes #21304.