complibs: add ISL

ISL is used by gcc-4.8 onward for GRAPHITE, so is also used as
backend for CLooG 0.18.0 onward.

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

# windows config options

## depends on ARCH_x86

##

## select WINDOWS

##

## help Build a toolchain targeting systems running Windows as host