Any derivative work is covered by the FLTK license and that include the additional permissions. It is my understanding that you cannot change the license at all unless it is explicitly permitted and I do not find this permission (I think this is the reason that when the FSF give extra permission, as it sometimes do, it clearly states you can remove the extra permission; otherwise the same problem would occurs).
Correct - you can't change the permissions on the work THAT WAS LICENCED TO YOU unless you are given permission (which is *very* *rarely* done)
If the FLTK demands that you use the FLTK for your own work, then that is unusual, and certainly demanding far more than the GPL (see below).
Moreover the LGPL sates: [ For example, if you distribute copies of the library, whether gratis or for a fee, you must give the recipients all the rights that we gave you ] This clearly suggests you must give the extra permissions to derivative works.
I'm not at all sure it does. Think about mixing LGPL and GPL code. The resulting work has (effectively) had its LGPL rights stripped. But there's nothing preventing the recipient separating the GPL and LGPL parts and using each according to its licence.
If, however, the FLTK does explicity require you to give the extra permissions, then it is GPL (and LGPL?) incompatible.
Look at this way. The GPL *DOES* *NOT* *EVER* make you licence your code under the GPL, even if you mix it with someone else's GPL code and distribute it. What it does is require you to licence your code under a GPL-compatible licence, which guarantees to the recipient that they can *safely* treat the entire work *AS* *IF* it were GPL-licenced.
What is the FLTK trying to achieve? The guarantee provided by the GPL is that, as a recipient, you do not need to care what the licence is on the individual bits. If ANY of it is GPL, you can safely behave *as* *if* *all* of it is GPL, even if it isn't.
Cheers, Wol -- Anthony W. Youngman - anthony@thewolery.demon.co.uk