ATTENTION: is the way DESTDIR was introduced completely wrong?

[Andrew Pantyukhin]

So now that Dmitry sounded a sober thought, I'll give a one
of my own.

I would rather see PREFIX respect DESTDIR and go through
all the pains of respecting PREFIX. This way we reach a
double cause:

* ensure PREFIX is respected (this will reveal thousands of
erring ports, but it will be worth it)
* ensure DESTDIR is respected

Also, we won't need to s/PREFIX/TARGETDIR/ anymore, a
feat of quite questionable usefullness.

For reasons that Dmitry highlighted, we'll need PREFIX_REL
and some additional _REL's.