The simple git-version script asks git for the current
revision, and falls back to 'unknown' if the result is
empty. This will have little effect until we have tags
in the repository.

We use the FILE_VERSION_FILTER option on doxygen to
embed this in the documentation with each file reference.

Unfortunately, there doesn't seem to be a way to
set PROJECT_NUMBER globally in the same way. This
commit unsets that option to avoid confusing references
in the generated documentation.