I should have caught this during review but the feature was too cool and
I didn't really pay attention (sorry). For consistency with the rest of
the scripts here.
This allows us to rebuild the manpages and html documentation only when
necessary. It is not necessary to manually keep the list of dependencies
up to date.
Unfortunately rst2pdf does not yet support this feature, see
https://github.com/rst2pdf/rst2pdf/issues/1108
