It has been observed that the --prefix option in ./configure (which puts
the value in $PREFIX) isn't actually referred to anywhere.

I'm not sure if this maybe means that it ought to be dropped?  Or
documented as being useless?

Certainly NOT a 1.1.x matter; if there's something to fix, it ought to
be done in HEAD...

At any rate, if it's intentionally vestigal, then, like the appendix, we
should probably keep it.  But if it is truly irrelevant, it otta get
dropped so that people don't get confused as to what it's there for...