/usr/lib/libmgl-fltk.so is in libmgl-dev 2.2.2.1-3+b2.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/libmgl-fltk.so.7.2.0
/usr/lib/libmgl-fltk.so.7.2.0