These are required by OWL tools to use vocabulary from another ontology.
Though we don't use OWL tools in LV2, they are at least useful for sanity
checking the semantic side of things.  These properties are also nice to have
anyway, since they clearly describe the dependencies between specifications.

For consistency, and because clang-tidy can't figure out if a define is an
include guard unless it ends the file.

Not all of these actually show up in the output, but it's unnecessary to use
these in Markdown anyway.