complibs/cloog: add support for the ISL backend

CLooG 0.18+ will use ISL instead of PPL, so we have to configure
adequately depending of which backend is in use.

The Kconfig entries will decide for us which is selected, so we
can rely on either PPL xor ISL to be selected, not both.

Reported-by: "Plotnikov Dmitry" <leitz@ispras.ru>
[Dmitry did a preliminray patch to add ISL support,
which this patch is inspired from]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>

 COPYING