(toplevel) introduce host subdir configuration in Makefile

[Nathanael Nerode]

>>[...] This particular bit is likely to be the most destabilizing of my 
>>changes; most of the stuff prior to it was carefully
>> designed to be "behavior-change-free".
> Yes, that's why I'd like to see it sooner rather than later. Plenty of 
>time before 5.4/6.0 branches to fix problems.
>
> Andrew
And when is that branch anticipated to happen?  (So I can plan...)

--Nathanael