I have a native FLTK port started, but it’s not finished. See here.
Well, yes and no (although it can sometimes be useful) :
Because libXpm. Yes, I needed libXpm for my (native!) XEmacs port and this crap requires all of the X11 libs. 
The other option is to patch it like they did for AmigaOS but it’s ugly and I’m not sure it’d be accepted upstream. Or maybe if it’s done in a clean enough way…