counterpart to X11BASE (default "/usr/X11R6").
Now PREFIX is set to ${X11BASE} or ${LOCAL_PREFIX} depending on
whether USE_IMAKE or USE_X11 is set or not.
This enables us to refer to non-X ports from X ports using
${LOCAL_PREFIX}, thus removing most of the remaining "/usr/local"s
from the ports tree.
This will also allow the system administrator to move the whole
"local" tree to somewhere else, without affecting X ports. (Of course
not all ports are necessarily happy with that, but we're working on
it.)
Based on: an idea that came up while I was watching a football game
several months ago ("hey, maybe I can move that sideline
without disturbing the other!")
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