How to define variables when using portupgrade

[Adam]

When compiling a port for the first time, you can define variables for
the Makefile easily:
make -DWITH_GMP install clean

However, I'm not sure how to define the same variable when using
portupgrade. Is it possible? I am currently manually deinstalling and
recompiling ports whenever I need to customize the compile process.

Is there a better way?

Thanks,
-- 
Adam <blueeskimo at gmx.net>