fixed a bug on systems that cannot populate FLTK_INCLUDE_DIRS

When compiling indigo-full on Gentoo, my `FindFLTK.cmake` (cmake version 2.8.12.2) did not populate the `FLTK_INCLUDE_DIRS` variable, resulting in a compile error. This patch adds a harmless statement that will try to populate `FLTK_INCLUDE_DIRS` from `FLTK_INCLUDE_DIR` only if it is empty.

```
-- Found FLTK:
/usr/lib64/fltk-1/libfltk_images.so;/usr/lib64/fltk-1/libfltk_forms.so;/usr/lib64/fltk-1/libfltk_gl.so;/usr/lib64/libGL.so;/usr/lib64/fltk-1/libfltk.so  
-- FLTK_LIBRARIES=/usr/lib64/fltk-1/libfltk_images.so;/usr/lib64/fltk-1/libfltk_forms.so;/usr/lib64/fltk-1/libfltk_gl.so;/usr/lib64/libGL.so;/usr/lib64/fltk-1/libfltk.so;/usr/lib64/libSM.so;/usr/lib64/libICE.so;/usr/lib64/libX11.so;/usr/lib64/libXext.so;/usr/lib64/libm.so
-- FLTK_INCLUDE_DIR=/usr/include/fltk-1;/usr/include
-- FLTK_INCLUDE_DIRS=
```

```
In file included from
/home/wayne/work/ros/ros_catkin_ws/src/stage_ros/src/stageros.cpp:38:0:
/opt/ros/indigo/include/Stage-4.1/stage.hh:57:19: fatal error: FL/Fl.H: No
such file or directory
compilation terminated.
make[2]: *** [CMakeFiles/stageros.dir/src/stageros.cpp.o] Error 1
make[1]: *** [CMakeFiles/stageros.dir/all] Error 2
make: *** [all] Error 2
<== Failed to process package 'stage_ros': 
```