CMake: Deprecate FLTK_USE_FILE (UseFLTK.cmake)

Cherry-pick the essential changes from FLTK 1.3 since this change
had not been ported to 1.4 yet.

To do: my current plan is to consolidate 1.3 and 1.4 CMake files
as far as possible (with the exceptions of source files, obviously)
and to redesign / refactor CMake files later in this process or
maybe only for 1.4 if it turns out to be too much to backport.