[tex4ht] dependencies, new and incomplete

[Karl Berry]

Pursuant to Michal's comment at https://puszcza.gnu.org.ua/bugs/?217.

1) I'm afraid I don't really understand tex4ht file derivation, despite
Deimi's best efforts to explain.

2) Nevertheless, by grepping the logs, I see that nearly everything
depends on mktex4ht.4ht and tex4ht.sty. I just added those as common
dependencies (except for the files which generate them). Hope it helps
and doesn't break anything. Easy enough to revert/tweak, of course.

I admit I'm doubtful that the particular case Michal writes about
(adding a new html4-related entry to mktex4ht-cnf.tex) is completely
addressed.

3) It sure would be nice to have complete auto-generated dependencies,
as we were discussing some time ago. Maybe someday ...

Now, must get back to another neglected task :( ... -k