GNU make problem with environment variables on IRIX

[Charlie Zender]

Following is a three line Makefile which is processed differently 
by GNU make version 3.79.1 on GNU/Linux vs. IRIX systems. I'm pretty
sure, but not positive, that the linux behavior is correct, but
make syntax is so bizarre maybe there's something I don't understand.

Here's the Makefile.tst, then the results on GNU/Linux then on IRIX:

# Purpose: Makefile demonstrates problem with environment variable
expansions
# Results differ on IRIX gmake 3.79.1 vs. GNU/Linux 3.79.1
# IRIX version prints value of $HOME environment variable and ignores
Makefile modification
# Linux version prints $HOME environment variable as modified in
Makefile.tst
# Usage: make -f Makefile.tst
HOME := ${HOME}_plus_foo_set_in_Makefile
tst:
        @printf "HOME = ${HOME}\n"

address@hidden:~/mk$ uname
Linux
address@hidden:~/mk$ echo $HOME
/home/zender
address@hidden:~/mk$ make -f Makefile.tst
HOME = /home/zender_plus_foo_set_in_Makefile

address@hidden:~/mk$ uname
IRIX64
address@hidden:~/mk$ echo $HOME
/home/ess/zender/zender
address@hidden:~/mk$ make -f Makefile.tst
HOME = /home/ess/zender/zender

As you can see, the IRIX version of gmake does not modify the 
$HOME environment variable at all. This is a big problem for me.
Is there a known workaround for IRIX? Can others duplicate this
problem?

Thanks,
Charlie
-- 
Charlie Zender address@hidden (949) 824-2987/FAX-3256, Department of
Earth System Science, University of California, Irvine CA 92697-3100