Bug#907920: use optional PETSc dependency

[Santiago Vila]

reassign 907920 gmsh
thanks

On Tue, Sep 04, 2018 at 07:57:55AM +0200, Nico Schlömer wrote:
> Package: hello
> Version: 3.0.6+dfsg1-3
> 
> 
> When building some meshes, I'm getting
> ```
> Error : Petsc is required (we do need complex in discreteFace::crossField())
> ```
> followed a double free or corruption. See, e.g., <
> https://github.com/nschloe/pygmsh/pull/214>.
> 
> Please include the dependency on PETSc.

I guess the submitter meant "Package: gmsh" here.

Thanks.