I found that everything we test on travis-ci is built with -O0.
I believe this is the wrong thing to do as we test something that is
compiled not the way it would be compiled for any real use.

Drop enabling DEBUG for GCOV.

Signed-off-by: Kir Kolyshkin <kir at openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>