I found today that making packages for X11 ports wasn't working.
Further investigation showed that prefix was erroneously set to /usr/local
for X11 based ports as well, when the assumption was that they'd really go
into ${X11BASE} (an /etc/make.conf variable that the user's free to set).

Set X11BASE to /usr/X11R6 if the user hasn't already, and assume that
the user really wants prefix to point there when the port is XMKMF based.