/usr/lib/x86_64-linux-gnu/libgdkglextmm-x11-1.2.so is in libgtkglextmm-x11-1.2-dev 1.2.0-6ubuntu1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libgdkglextmm-x11-1.2.so.0.0.0