(toplevel) introduce host subdir configuration in Makefile

[DJ Delorie]

> Hmm...  I thought the whole point of having explicit configure
> targets was to be able to run them in parallel.

If we can arrange that, that would be better.  For now I think just
serializing the configures (allowing builds in parallel) will be a
sufficient gain.

> Of course we'll still be able to configure a sub-package while
> another builds, but that's not quite as good.

It will also allow re-configuring to pick up one or two new
subdirectories (or manually de-configured) without reconfiguring
everything.