It occurred to me that Jeff Rizzo wrote in gmane.os.netbsd.current:
>
>
> On Sun, Jun 05, 2005 at 10:08:36AM -0700, Hisashi T Fujinaka wrote:
>> I know that linking things willy-nilly for /usr/src makes big problems,
>> because I've been through this previously. However, I put src in
>> /var/src and I'm linking to /usr/src. 
>
> Have you set NETBSDSRCDIR to /var/src ?  (setting BSDSRCDIR should work,
> too)

Hm, I have my src in another directory as well (Having multiple NetBSD
machines, some of them low on diskspace, it makes sense to have src, xsrc
and pkgsrc shared over NFS, so in order to make things a little more 
maintainable, I've grouped these trees together in a subdir), and never
had to set NETBSDSRCDIR as long as I kept a symlink in place..