> Hmm, this also works fine here (on -current, with a basically empty
> /etc/mk.conf):
Ah. A few second later I've understood it.
After removing MAKEOBJDIR=xxx line from mk.conf,
it seems it works now.

It looks like some mk.conf variables conflict with build.sh arguments.
It is very strange IMHO and should be documented in the NetBSD Guide.

-- 
Best regards, Aleksey Cheusov.