Passing an argument to a port

[Mark Edwards]

Can someone clue me in on how to pass a variable to a port during make?

I am trying to customize the behavior of mail/imap-uw and mail/cclient. 
  They ordinarily use unix mailbox format when creating new mailboxes, 
and I want them to create mbx format mailboxes.  This is achieved by 
setting CREATEPROTO=mbxproto during compilation.  For example:

make osx CREATEPROTO=mbxproto

achieves this on my OSX machine.  However, I'm not sure how to pass 
this setting to the FreeBSD port.  If I do:

cd /usr/ports/mail/imap-uw
make install CREATEPROTO=mbxproto

it doesn't happen.  Is there some way to pass a given argument to the 
make process from the make command in the port?

Thanks!

--
Mark Edwards