[CMake] Finding FLTK?

[Christian Convey]

I'm using FindFLTK, and I seem to need to specify, on my own:
    FLTK_INCLUDE_DIR=/usr/include/FL

If I don't set it explicitly, then the variable FLTK_FOUND ends up
having a value of 0 rather than 1.

Does anyone know why this is?  When I look through the shipped version
of FindFLTK.cmake, line 179 appears to specifically indicate that the
module author expected FLTK's header files to reside in a "FL"
subdirectory:

    SET(FLTK_INCLUDE_SEARCH_PATH ${FLTK_INCLUDE_SEARCH_PATH}
      /usr/local/include
      /usr/include
      /usr/local/fltk
      /usr/X11R6/include
      )

    FIND_PATH(FLTK_INCLUDE_DIR FL/Fl.h ${FLTK_INCLUDE_SEARCH_PATH})

Unfortunately that module's code is pretty hard for me to follow, so
I'm not sure why the code shown above either doesn't run or doesn't
work.

Thanks,
Christian