Googling .. fltk-config
http://www.fltk.org/doc-1.1/basics.html
And have a look in some applications using fltk,
e.g. xpp, OpenEXR, SpiralSynth, Gmsh,
alsamixer-gui, ice_examin, prozgui.
I'd guess, you can find one using fltk-config in
one of the Makefile's.
( xpp has it mentioned after running configure )
....