/usr/lib/x86_64-linux-gnu/libIlmThread.so is in libilmbase-dev 2.2.0-12.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libIlmThread-2_2.so.12.0.0
/usr/lib/x86_64-linux-gnu/libIlmThread-2_2.so.12.0.0