/usr/lib/x86_64-linux-gnu/libfltk.so is in libfltk1.1-dev 1.1.10-17.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libfltk.so.1.1
/usr/lib/x86_64-linux-gnu/libfltk.so.1.1