Follow-up from "Split config file into public and private config files"

The following discussion from !1262 (merged) should be addressed:

• @mathis.kelm started a discussion: (+1 comment)

  @santiago.ospina in DuMux we use definitions like HAVE_DUNE_FOAMGRID in our headers. With this change, we've noticed that for dependencies without a config.h.cmake file (e.g. dune-subgrid, dune-foamgrid) these definitions are no longer added to the config.h file generated for our module. The function dune_parse_module_config_file is not called for dependencies without config.h.cmake so neither the detection nor the public headers are included in the legacy config header of downstream modules. The new config header include/dumux-config.hh does try to include public config headers of the dependencies in question, but those are not generated.

  Are all DUNE modules expected to have a config.h.cmake file now or should at least the detection of dependencies still be included in downstream config headers? Should we make changes in our module?