set permissions and ownerships of PREFIX (usually /usr/local). This
is the default if USE_IMAKE or USE_X11 is set.
This should be useful for machines like thud, where we want to keep
the /usr/local subtree writable to a group ("ports" in our case). Anybody
who installs stuff in /usr/local should have this set in the environment.
Note this won't affect anything the pkg_* suite does.