Install libfltk1.3-compat-headers by entering the following commands in the terminal:
sudo apt update sudo apt install libfltk1.3-compat-headers
Description:
Fast Light Toolkit - compatibility header symlinks
This package provides header symlinks with .h rather than .H extensions for compatibility with some older code that relies on the lowercase-h names.
Homepage: http://www.fltk.org/
Version: 1.3.4-6
Section: universe/libdevel