I have just verified that the same object file, hello.o, output by command

    g++ -I/path/to/source/FLTK/tree -c hello.cxx

can be linked to the dynamic libfltk libraries built with

OPTION_APPLE_X11 set either off or  on.

That provides a scenario where an app could test the values of Fl::cfg_gfx_quartz

or Fl::cfg_gfx_xlib.