> /usr/obj directory. 
I presume you remembered to create this directory
and that you can write to it.
> I set 'BSDOBJDIR = /usr/obj' in  '/usr/src/share/mk/bsd.own.mk' and
I set mine in /etc/mk.conf because that gets included and my
changes are not in the way when I run make install from src/share/mk.

Anyway this is where you went wrong, you forgot to install bsd.own.mk
in /usr/share/mk

Cheerio

PS an example /etc/mk.conf is appended, why the MAKE definition is
ignored I don't know.
# NetBSD local mk-configuration options
.if !defined(ETCMKCONF)
ETCMKCONF=/etc/mk.conf
BSDSRCDIR=/src/src
BSDOBJDIR=/mit/obj
CFLAGS+=-O6 -fno-strength-reduce -m486 -pipe
MAKE+=	-B -j 4
.endif

-- 
The cost of living hasn't affected its popularity. (Mark Twain)
current	release=doc host=netbsd.wifo.uni-mannheim.de \ "NetBSD-current mirror"
hostbase=/mit/ftp/pub/NetBSD base=/usr prefix=/usr backup delete use-rel-suffix