FLTK 1.5 is based on the release of FLTK 1.4.2. Later updates and
bug fixes may be backported to 1.4.x (x > 2).

FLTK 1.5 is intended to be mostly API compatible with FLTK 1.4.x so
you don't need to change source code when you switch to FLTK 1.5.
However, all programs must be recompiled with FLTK 1.5 because the
ABI (Application Binary Interface) has changed.

More info TBD.
