[Cmake] Optionally use a system isl version.

This patch adds an option to build against a version of libisl already
installed on the system. The installation is autodetected using the
pkg-config file shipped with isl.

The detection of the library is in the FindISL.cmake module that creates
an imported target.

Contributed-by: Philip Pfaffe <philip.pfaffe@gmail.com>

Differential Revision: https://reviews.llvm.org/D30043

llvm-svn: 296361