Install libfltk1.1-dev by entering the following commands in the terminal:
sudo apt update sudo apt install libfltk1.1-dev
Description:
Fast Light Toolkit - development files
This package provides the files necessary for compiling programs that depend on FLTK. . The Fast Light Toolkit (FLTK) is a cross-platform graphical user interface toolkit originally based on libForms.
Homepage: http://www.fltk.org/
Version: 1.1.10-23
Section: universe/libdevel