Add build_with support for dolfin (like we have for petsc).

This will add `build_with` support for `dolfin` (like we have for `petsc`). One can now do for instance:

    dolfin:
        build_with: |
          suitesparse, cgal, scotch, parmetis, petsc, petsc4py, slepc, trilinos, pastix, zlib, hdf5, vtk
        build_type: 'RelWithDebInfo'

Note also the `build_type` parameter, which sets the `CMAKE_BUILD_TYPE` used in a build. For `dolfin`, this could be one of `Release`, `Debug`, `RelWitDebInfo` or `Developer`.