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>