There's a merged pull request on the void-packages repository

isl, isl15, cloog: remove bootstrap=yes
https://github.com/void-linux/void-packages/pull/33332

Description:
It seems gforge.inria.fr is down.

Per README, the project repo is on repo.or.cz, but the original tarballs
are not available. The available tarballs need autotools to build.

Also removed 'bootstrap=yes'.