Fix html docs builds

This fixes the doc builds broken by commit b292b986, but is only an
ugly workaround due to our dependency chain.

opcodes.py imports (both directly and indirectly) constants.py, which
imports _autoconf.py. Hence one could say that all .py files have a
logical dependency on _autoconf.py, since you can't "import opcodes"
before _autoconf.py is built. So the sphinx rule should depend on
_autoconf.py as a prerequisite.

But we can't use a normal prerequisite, since that would cause the
html docs to be rebuilt whenever _autoconf.py gets rebuilt, i.e. even
on end-users machines after ./configure; this is bad, and it also
breaks distcheck (as the html files are not removed in distclean,
since we do want them distributed).

So we only add an order-only prerequisite, which does the right thing
(although our dependency chain is still ugly).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Adeodato Simo <dato@google.com>