CMake: correct own error using undefined var

CMakeLists.txt

@@ -178,6 +178,15 @@ if(UNIX AND NOT APPLE)
 	set(_init_GAMEENGINE                     OFF)
 endif()
 
+
+#-----------------------------------------------------------------------------
+# Options
+
+# First platform spesific non-cached vars
+if(UNIX AND NOT APPLE)
+	set(WITH_X11 ON)
+endif()
+
 # Blender internal features
 option(WITH_BLENDER "Build blender (disable to build only the blender player)" ON)
 mark_as_advanced(WITH_BLENDER)
@@ -233,10 +242,6 @@ mark_as_advanced(WITH_AUDASPACE)
 
 option(WITH_OPENMP        "Enable OpenMP (has to be supported by the compiler)" ON)
 
-if(UNIX AND NOT APPLE)
-	set(WITH_X11 ON)
-endif()
-
 if(WITH_X11)
 	option(WITH_X11_XINPUT    "Enable X11 Xinput (tablet support and unicode input)"  ON)
 	option(WITH_X11_XF86VMODE "Enable X11 video mode switching"                       ON)