Requiring USE_DESTDIR=yes after the branch

[Joerg Sonnenberger <joerg%britannica.bec.de@localhost>]

Hi all,
following the pkgsrc-2015Q3 branch, I would like to remove the support
for USE_DESTDIR=no from pkgsrc. In terms of the infrastructure, the only
thing still depending on it is bootstrapping. I am working on that, but
I've been running out of time before the freeze started. Otherwise a
proper warning would have been added already.

Joerg