This is because Gen programs are statically linked, and static linking is a derivative work unless otherwise noted. It would be good to clarify this, for example by adding an exception to the readme similar to FLTK's exception:
http://www.fltk.org/COPYING.php
Static linking of applications and widgets to the FLTK library does not constitute a derivative work and does not require the author to provide source code for the application or widget, use the shared FLTK libraries, or link their applications or widgets against a user-supplied version of FLTK.
If you link the application or widget to a modified version of FLTK, then the changes to FLTK must be provided under the terms of the LGPL in sections 1, 2, and 4.
You do not have to provide a copy of the FLTK license with programs that are linked to the FLTK library, nor do you have to identify the FLTK license in your program or documentation as required by section 6 of the LGPL.