As per https://repo.or.cz/isl.git/blob_plain/HEAD:/LICENSE and https://libisl.sourceforge.io/ isl is licensed as MIT not LGPL. Reproducible: Always
Looks like it used to be LGPL-2.1: https://repo.or.cz/isl.git/commit/6bcdd73ae1efb847a94e5dd003c95133016e9685. It changed in 2012 to MIT: https://repo.or.cz/isl.git/commit/056289f285e62c52cc59ee826172e4d3092ef3fe.