using build.sh in src subdirectories

[Taylor R Campbell <campbell%mumble.net@localhost>]

Sometimes I want to build just parts of NetBSD's source tree.  For
example, if an OpenSSH security advisory is released, I might want to
build and install src/usr.bin/ssh/, but nothing else.  However, I also
want to keep my object and tool directories separate from my source
directory.  build.sh has a convenient way to specify object and tool
directories -- for an entire build or an entire install.  But if I
want to specify this while making in only a subdirectory, the only way
I know to do that is to run `build.sh params' and painstakingly copy
all of the relevant parameters into an invocation of make(1) in the
subdirectory.

Am I missing an obvious way to tell build.sh to make some target in a
subdirectory, or give me a shell with all the relevant environment
variables set up so that I can run make(1) (or the wrapper therefor)
myself in a subdirectory?  If not, am I missing something about why
not, or would this be something worth implementing?

Handling destdirs and installworlddirs might pose an issue for
installing from a single source directory -- I don't know anything
about how this works, but I imagine that it might take some effort to
persuade build.sh to copy just those files from the destdir to the
installworlddir that are relevant, since simply making the install
target in the desired subdirectory will install files into the
destdir, not into the installworlddir.