COMP: Fix configuration without explicitly setting CTK_QT_VERSION

Address a regression introduced in a0e74a0 (COMP: Update CMake files
removing support for Qt 4.x) by fixing the following error reported when
configuring without explicitly setting CTK_QT_VERSION:

```
cmake \
  -DQt5_DIR:PATH=$Qt5_DIR \
  -DCMAKE_BUILD_TYPE:STRING=Release \
  ../CTK

[...]
-- Checking if --no-as-needed linker flag is required - no
-- CTK_LIB_Core_WITH_BFD_SHARED is OFF
-- CTK_LIB_Core_WITH_BFD_STATIC is OFF
CMake Error at Libs/Visualization/VTK/Widgets/target_libraries.cmake:15 (message):
  Support for this Qt is not implemented
Call Stack (most recent call first):
  CMake/ctkMacroTargetLibraries.cmake:131 (include)
  CMake/ctkFunctionGenerateDGraphInput.cmake:78 (ctkFunctionCollectTargetLibraryNames)
  CMakeLists.txt:844 (ctkFunctionGenerateDGraphInput)
[...]
```

CMake/ctkMacroSetupQt.cmake

@@ -20,14 +20,6 @@
 
 #! \ingroup CMakeUtilities
 macro(ctkMacroSetupQt)
-  set(CTK_QT_VERSION "5" CACHE STRING "Expected Qt version")
-  mark_as_advanced(CTK_QT_VERSION)
-
-  set_property(CACHE CTK_QT_VERSION PROPERTY STRINGS 5)
-
-  if(NOT CTK_QT_VERSION VERSION_EQUAL "5")
-    message(FATAL_ERROR "Expected value for CTK_QT_VERSION is '5'")
-  endif()
 
   if(CTK_QT_VERSION VERSION_EQUAL "5")
     cmake_minimum_required(VERSION 2.8.12)
@@ -139,6 +131,4 @@ macro(ctkMacroSetupQt)
   else()
     message(FATAL_ERROR "Support for Qt${CTK_QT_VERSION} is not implemented")
   endif()
-
-  mark_as_superbuild(CTK_QT_VERSION)
 endmacro()

CMakeLists.txt

@@ -102,6 +102,14 @@ mark_as_superbuild(
 option(CTK_SUPERBUILD "Build ${PROJECT_NAME} and the projects it depends on." ON)
 mark_as_advanced(CTK_SUPERBUILD)
 
+#-----------------------------------------------------------------------------
+# Qt version
+#
+set(CTK_QT_VERSION "5" CACHE STRING "Expected Qt version")
+mark_as_advanced(CTK_QT_VERSION)
+set_property(CACHE CTK_QT_VERSION PROPERTY STRINGS 5)
+mark_as_superbuild(CTK_QT_VERSION)
+
 #-----------------------------------------------------------------------------
 # Output directories.
 #