[!275] [cmake] Allow code generation system tests to depend on additional files

Merge branch 'feature/cmake-allow-more-generation-dependencies' into 'master'

ref:dominic/dune-perftool I have the use case of a boundary condition file,
whose changes should retrigger the code generation process.

See merge request [dominic/dune-perftool!275]

  [dominic/dune-perftool!275]: gitlab.dune-project.org/dominic/dune-perftool/merge_requests/275