Creating pkg-plist - problems

[vittorio]

Under 6.1 I'm trying to build a port (R-2.3.1) following the instructions in 
the porters-handbook and particularly I'm having a go at creating the 
pkg-plist file as suggested in the point 7.5 of

http://www.freebsd.org/doc/en_US.ISO8859-1/books/porters-handbook/plist-autoplist.html

now,there is said:
 
"Next, create a temporary directory tree into which your port can be 
installed, and install any dependencies.
# mkdir /var/tmp/$(make -V PORTNAME)
# mtree -U -f $(make -V MTREE_FILE) -d -e -p /var/tmp/$(make -V PORTNAME)
# make depends PREFIX=/var/tmp/$(make -V PORTNAME)"

but, when I try:

% make -V PORTNAME
R

BUT
% mkdir /var/tmp/$(make -V PORTNAME)
the answer is 
"Nome di variabile non lecito" that is "variable name not allowed"

What's wrong with it?

Ciao
Vittorio