Problems Installing on FreeBSD4.x using make cmd line variables

I am having problems installing nullmailer-1.00 in a
command-line-specified directory on FreeBSD4.11 when running `make
install prefix=/tmp/nullmailer`. I performed the
configure/make/install process using both /bin/sh and /bin/csh shells
with the same results. As a test, I performed this exact process using
sudo source code and it worked as expected, so I'm pretty sure it's
not my make binary or configs.

The process outlined below works perfectly in Slackware and OpenBSD.

# ./configure --prefix=/usr/local
# make
# make install prefix=/tmp/nullmailer
(at this point everything is under /usr/local/)
# make install-root
(target, install-root, fails because it cannot find files in /tmp/nullmailer)

WORKAROUND (csh)

# ./configure --prefix=/usr/local
# make
# setenv prefix "/tmp/nullmailer"
# make -E prefix install
# make -E prefix install-root

I guess I'm just curious why make's command line variables are
essentially ignored, while it acknowledges environment vars?

- pachl