/usr/lib/x86_64-linux-gnu/libIlmThread.so is in libilmbase-dev 1.0.1-6ubuntu1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libIlmThread.so.6.0.0
/usr/lib/x86_64-linux-gnu/libIlmThread.so.6.0.0