Enable HTML manual building by default but for distribution tarballs

Enable building of the HTML manual by default unless not building from
Git and with an existing local copy (which is included in distribution
tarballs).

This makes sure we can install the HTML manual without having it
checked in VCS, yet not require rst2html for tarball builds.
Cleanup doc Makefile so that each conditional set all its targets

Don't set clean-local and all-local dependencies all together at the
end and rather let each section add the appropriate dependencies.
This makes each conditional more self-contained, and is safe as Make
allows adding extra dependencies to existing targets.
Don't clean HTML manual if we didn't build it

Clean the HTML manual upon 'maintainer-clean' rather than 'clean' in
case it was not built by Make but rather part of the distribution.
This is fine even then, as configure will properly require what is
needed to build it again if it is missing.