Hi,

This patch adds the optional dependency Trilinos to dealii-openmpi.
(Starting from deal.II 10.0.0, the Trilinos interface will require
MPI, and anyway parts of deal.II's Trilinos interface do not compile
properly without MPI even in earlier versions.)

This patch is necessary for me to package Lethe [1] next, which
requires deal.II to be compiled with Trilinos support.

Best regards,
Paul

[1]: https://github.com/lethe-cfd/lethe