...fltk-1.3.6rc1/test$ mv checkers.app/ checkers_original.app
...fltk-1.3.6rc1/test$ mkdir -p checkers.app/Contents/Resources/
...fltk-1.3.6rc1/test$ cp checkers_original.app/Contents/Info.plist checkers.app/Contents/
...fltk-1.3.6rc1/test$ cp checkers_original.app/Contents/Resources/checkers.icns checkers.app/Contents/Resources/
...fltk-1.3.6rc1/test$ cp checkers_original.app/Contents/PkgInfo checkers.app/Contents/
...fltk-1.3.6rc1/test$ rm checkers
...fltk-1.3.6rc1/test$ make
Linking checkers...
...fltk-1.3.6rc1/test$ open checkers.app/

WORKS!