Change name of variable

Tests/X11/CMakeLists.txt

@@ -7,6 +7,6 @@ ADD_EXECUTABLE (UseX11 X11.c)
 
 IF(CMAKE_HAS_X)
   ADD_DEFINITIONS(-DCMAKE_HAS_X)
-  INCLUDE_DIRECTORIES(${CMAKE_X_INCLUDE_DIRS})
+  INCLUDE_DIRECTORIES(${X11_INCLUDE_DIR})
   TARGET_LINK_LIBRARIES(UseX11 ${CMAKE_X_LIBS})
 ENDIF(CMAKE_HAS_X)