Update to isl 0.25

[bittor7x0]

The official isl Git repository tagged isl-0.25 here 4 months ago, but their snapshots files only include the commit id, which hides the version actually used.

So we download isl from a GitHub mirror (its owner says here "This repository is a mirror of the official isl repository"), which does not have isl 0.25 tagged (the latest tagged version is 0.24) and therefore we are unable to update to this version from this mirror.

The official isl website is usually down, so we can't use it to update to isl 0.25.

The author of isl is Sven Verdoolaege and he's publishing isl releases on SourceForge:
https://sourceforge.net/projects/libisl/files/
https://libisl.sourceforge.io/

He commented here "I actually published my releases on the web site hosted by sourceforge, but I guess you want me to put them in files as well", so we will use this last URL to update isl.