gcc: support grahite only for GCC 5 or above

GCC 4.9.x requires an old version of ISL to support graphite, and this
old version of ISL is not sufficient to support graphite in modern GCC
versions.

Since GCC 4.9.x is getting older, and to keep things simple, we stop
supporting graphite on old GCC < 5.x. This way, we will be able to
bump ISL to a version that is suitable for modern GCC versions.

In addition, this allows to drop the dependency on cloog, which was
only needed for GCC 4.9.

Signed-off-by: Andrey Yurovsky <yurovsky@gmail.com>
[Thomas: rework commit log.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>