Conditional Support for SuperLU-Dist
------------------------------------

In principle dolfin supports SuperLU-Dist. However the superlu-dist
package distributed by Debian is mostly useless since mc64, one of its
core functions, cannot legally be redistributed and so has been
removed from the Debian package. For this reason, we have given
preference to mumps over superlu_dist.

You may specify superlu_dist if desired (or if you have a local build
of superlu_dist that includes mc64), for instance with 
  solve(a == L, u, bc, solver_parameters={'linear_solver': 'superlu_dist'})

You may also successfully use superlu_dist without mc64 by specifying
C++:
  PETScOptions::set("-mat_superlu_dist_rowperm", "NATURAL");
Python:
  PETScOptions.set('-mat_superlu_dist_rowperm', 'NATURAL')
Performance will likely be reduced.

The same problem applies to superlu, from which mc64 has also been
removed. superlu still functions successfully however (mc64 is not
core for the serial version of SuperLU).
