How to make already defined make(1) variable undefined from command line

[Yasuhiro KIMURA]

Question:

Assuming that variable FOO is defined in Makefile or /etc/make.conf or
somewhere else, are there any way to make FOO undefined from command
line?

Background:

I put 'DEVELOPER=yes' in /etc/make.conf. While it helps improving
quality of ports, sometimes I want to disable developer mode to stop
known warnings displayed. At first I tried following commands.

% make DEVELOPER=no
% make DEVELOPER=false
% make DEVELOPER=0
% make DEVELOPER=
...

But none of them worked as is expected because ports framework only
checks if DEVELOPER is defined and doesn't see its value at all. So
now I edit /etc/make.conf and comment out the definition each time but
it's a bother.

Then, are there any way to make already defined make(1) variable
undefined from command line?

Best Regards.

---
Yasuhiro KIMURA