add a specialized version of version compares that should not update

add a specialized version of version compares that should not update
everything all the time, at the bequest of aja.

(basically, we updated everything when runtime dependencies  changed, even
though we actually only care for wantlib, and hard dependencies, but those
trigger bumps all over the tree)

Experimental for now, turn on with -DSHORTENED. Likely to become the new
default if nothing bad comes out of it.