/usr/lib/x86_64-linux-gnu/libGLU.so.1 is in libglu1-mesa 9.0.0-2.1build1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libGLU.so.1.3.1
/usr/lib/x86_64-linux-gnu/libGLU.so.1.3.1