crit: fix 'make install'

Apparently, setup.py from distutils interprets --root= option without
an argument as "--root=." and we end up with what is described
in https://github.com/xemul/criu/issues/309

.

Fix is to prepend DESTDIR value (if any) to --prefix argument.

v2: fix uninstall as well
v3: same code, resent via gmail

Reported-by: Juraj Oršulić <juraj.orsulic@fer.hr>
Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>