#aptitude search fltk|grep dev
p libfltk1.1-dev - Fast Light Toolkit - development files
p libfltk1.3-dev - Fast Light Toolkit - development files
Why to choose one over another?
From version history:
This is basically FLTK-1.1.10 with extra features, but with an incompatible ABI. The new features include: full UTF-8 Unicode support, which allows left-to-right non-Latin text such as Greek and Cyrillic; new Fl_Tree, Fl_Table and Fl_Native_Filechooser widgets; printing support; a device abstraction layer; and a new progamming manual generated from the code using Doxygen.
As you see, you have now:
The OP also describes memory usage differences:
I also noticed 1.1 used 3 MB and 1.3 46 MB