portupgrade: undefining a variable in MAKE_ARGS set previously

[Naram Qashat]

When using portupgrade, I use the MAKE_ARGS variable inside of pkgtools.conf.  I 
have variables set for '*' as well as many ports.  However, I want to be able to 
undefine a variable I set previously in the '*' entry.  I found a mailing list 
thread from 2007 for rt36 that mentions using -U<variable> with make, but make 
complains saying that U is an illegal option, and the man page says nothing 
about -U being an option.  Even if it were an option, I'm not sure how BSD make 
would handle the situation of defining a variable with <variable>=<something> or 
-D<variable> and then later undefining it with -U<variable>.  So is there a way 
to do this either via pkgtools.conf or via make itself?

Thanks,
Naram Qashat