Polly (from LLVM) requires very specific versions of ISL and CLOOG.

Per the [project documentation](http://repo.or.cz/w/polly-mirror.git/blob/HEAD:/utils/checkout_cloog.sh), Polly is only tested against very specific versions of Cloog and ISL.  These versions can be found in a [script they use to checkout cloog](http://repo.or.cz/w/polly-mirror.git/blob/HEAD:/utils/checkout_cloog.sh).

This should fix homebrew/homebrew-versions#340.