the fullly conditionalised bsd.prefs.mk still needs a considerable
amount of time and moving the include guard out kills most of that.
Preliminary tests with renaming bsd.prefs.mk and using the same logic
to point to the real files show 5%-10% reduced run time for
"make show-options" in x11/kdebase3 and no measurable regression in
simpler places. This versions allows incremental conversion of the
use in buildlink3.mk. Some care should be used in mk/ itself, as e.g.
bsd.pkg.mk is included from first level hierachies as well.