/usr/lib/x86_64-linux-gnu/libisl.so is in libisl-0.18-dev 0.18-4.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libisl.so.15.3.0
/usr/lib/x86_64-linux-gnu/libisl.so.15.3.0