/usr/lib/x86_64-linux-gnu/libXm.so is in libmotif-dev 2.3.4-5.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libXm.so.4.0.4
/usr/lib/x86_64-linux-gnu/libXm.so.4.0.4