qt5: default to 5.8

CMakeLists.txt

@@ -309,6 +309,9 @@ set(_git_clone_arguments_default --progress)
 # set the default for cxx11 to be enabled for this project
 set(_superbuild_default_cxx11 ON)
 
+# set the default for qt5 to be 5.8
+set(_superbuild_qt5_default_selection "5.8")
+
 if (NOT EXISTS "${CMAKE_CURRENT_LIST_DIR}/superbuild/CMakeLists.txt")
   message(FATAL_ERROR "It appears as though the superbuild infrastructure "
                       "is missing; did you forget to do `git submodule init` "