Reversing dependency order

[DJ Delorie]

I've tried before to do this, but got lost in the source code looking
for the right spot.  I'd like to add an option that reverses the
dependency lists, so that things are built in reverse order except
when an explicit dependency requires otherwise.  In other words, if:

        a : b c
        a : d

It will build d, c, b; but if

        a : b c
        a : d
        b : c

it will build d, b, c.  Where is the right place to do this?

Having such an option would be very useful to verify that a worst-case
parallel build would work properly.