Changes to gtk-doc

[Damon Chaplin <damon ximian com>]

I just committed quite a few changes to gtk-doc, so let me know if
it causes any trouble. (Check that no docs disappear!)

It will now only update files if they have been changed, and it
outputs a few timestamp files.

This should hopefully make it very easy to create dependency rules
for building the HTML, so the HTML is only rebuilt when necessary.
(It may cut down on CVS traffic as well, since it won't touch the
tmpl/ files unless it has to.)

Damon