core: Require our included cloog/isl to be available

Summary:
For our newer cfX toolchains, this makes Polly support easier.

Depends on D460
Depends on D462

Test Plan: gtg if installed and run

Reviewers: randall.rushing, jriordan001, anders3408, synergy

Reviewed By: synergy

Differential Revision: http://review.codefirex.com/D461