Serialization dependencies muck up configure-on-demand

[DJ Delorie]

> We can use shell locking (*) in the top level to make sure no two
> configures run at the same time, if we really need that.

I really don't want to create a spinlock system just for a build.
There are too many weird ways that can fail.

>  Serialized dependencies are exactly the wrong way to solve the
> problem, IMO.  I hadn't realized they had been implemented like
> that, otherwise I'd have objected.

We've done it before.  We're using the "can't do this until you do
that" half of the dependency logic, not the "I need this built for my
build" half.