You may want to check also these:
https://www.fltk.org/str.php?L3513 - for FLTK 1.4.x.
https://discuss.haiku-os.org/t/fltk-port-was-port-debian-package-management-system-to-haiku/2996/37 - initial work by @mmu_man.
https://twitter.com/mmu_man/status/683275806429306884 - some screenshots.