Makefile, travis: unify checking GCOV env var

Setting GCOV environment variable is used for doing profiling (gcov).
Unfortunately, it is checked in two different ways:

1. In top Makefile and scripts/travis, it is checked against "1".
2. In other places, it's checked to be set to a non-empty value.

So, giving various values to GCOV will lead to different results.

Let's unify the approaches used. Apparently, among all the code the
easiest (and not too strict) way is to check for non-empty value
(this also leads to the smallest possible patch size).

Note that in Python, evaluating an empty string in a binary context
(such as after an "if" statement) equals to False.

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