building only part of the world

[perryh at pluto.rain.com]

How would I go about building, not the entire world, but only
a small part of it?

If I just cd to the desired subdirectory and type "make -n"
-- intending to find out what it would try to do -- I get a
warning about not having changed the object directory.

I suppose I'm supposed to type something along the lines of

  make -n OBJ=<something>

but what should I be setting OBJ to?  An attempt to find(1)
some of the expected output files, so as to discover where
they are conventionally located, found nothing.