/usr/lib/x86_64-linux-gnu/libSDL.so is in libsdl1.2-dev 1.2.15-5.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libSDL-1.2.so.0.11.4
/usr/lib/x86_64-linux-gnu/libSDL-1.2.so.0.11.4