Fix UseCTK.cmake to work for very simple project

This commit fixes regression introduced in 6cc6996 (Remove duplicates
from the include dirs) causing the Examples [1] to fail to configure
with the following error:

  CMake Error at /home/jcfr/Projects/CTK-Debug/UseCTK.cmake:34 (list):
    list sub-command REMOVE_ITEM requires two or more arguments.
  Call Stack (most recent call first):
    Example1/CMakeLists.txt:28 (include)


[1] https://github.com/commontk/Examples

CMake/UseCTK.cmake.in

@@ -30,7 +30,9 @@ if(NOT CTK_USE_FILE_INCLUDED)
   # Add include directories needed to use CTK.
   get_directory_property(existing_include_directories INCLUDE_DIRECTORIES)
   set(new_include_directories ${CTK_INCLUDE_DIRS})
-  list(REMOVE_ITEM new_include_directories ${existing_include_directories})
+  if(existing_include_directories)
+    list(REMOVE_ITEM new_include_directories ${existing_include_directories})
+  endif()
   include_directories(${new_include_directories})
 
   if (NOT DEFINED QT_QMAKE_EXECUTABLE)