(toplevel) introduce host subdir configuration in Makefile

[DJ Delorie]

> Now, on the other hand, we could configure GCC while building
> libiberty.  It would be nice to have a way to throw each configure step
> into its own log file so that they don't interleave on the screen,
> someday.

We could have each configured configure target depend on the previous
one in the list.  That should serialize them.  We can do this now
because configure has a specific list of which subdirs *will* be
configured, rather than the list of *potential* ones we used to have.

Assuming autogen can do that, of course ;-)