/usr/lib/x86_64-linux-gnu/libX11.so is in libx11-dev 2:1.5.0-1+deb7u2.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libX11.so.6.3.0
/usr/lib/x86_64-linux-gnu/libX11.so.6.3.0