Reintroduce the GNU semantics of DESTDIR
Reintroduce the GNU semantics of DESTDIR

This partially reverts commit a81cd747341e04783e7e3a94a76c4444c3bd5cd8
so that most path variables like PREFIX and BINDIR refer to paths on
the installed system, and DESTDIR is used only in (make install), following
the philosophy of the GNU coding standards for path variables. (But not precisely
the variable names, which are lowercase in the standard, nor the principle that
even SYSCONFDIR should be under $PREFIX.

Keep the use of ?= instead of = because it somewhat better expresses the idea
that the values can be overridden.

Use ${DESTDIR}${BINDIR} instead of ${DESTDIR}/${BINDIR} etc, so that a plain
(make install) does not use paths like //usr/bin/... ; strictly speaking they
are IIRC reserved by POSIX, and more importantly it just looks untidy :)

Signed-off-by: Miloslav Trmač <9c008e38982a5397deb855345fb164f0558459ae@redhat.com>

mtrmac/skopeo,mtrmac/skopeo,mtrmac/skopeo