[refactor] Update qt5 superbuild process.

This has been done to follow the steps to manage qt5 in the Paraview
Superbuild.

CMakeLists.txt

@@ -150,10 +150,17 @@ function (superbuild_add_packaging)
   endif ()
   list(GET generators 0 default_generator)
 
-  if (USE_SYSTEM_qt5)
+  if (qt5_enabled AND (USE_SYSTEM_qt5 OR APPLE OR WIN32))
     list(APPEND superbuild_export_variables
       Qt5_DIR)
+    find_package(Qt5 QUIET REQUIRED COMPONENTS Core)
+    set(qt5_version "${Qt5Core_VERSION_MAJOR}.${Qt5Core_VERSION_MINOR}")
+  else ()
+    set(qt5_version "${qt5_SOURCE_SELECTION}")
   endif ()
+  list(APPEND superbuild_export_variables
+    qt5_version)
+  
 
   # pass some variables that are required for packaging step:
   list(APPEND superbuild_export_variables