Prevent_deref shall not return a reference to the iterator as it can be copied

iterator being taken by copy usually, if a function takes a
an iterator by copy and that function returns the reference
type of the deref operator then a reference to a temporary
will be returned.

STL_Extension/include/CGAL/iterator.h

@@ -43,6 +43,8 @@ class Prevent_deref
   Prevent_deref<I>
   , I // base
   , I // value
+  , boost::use_default
+  , I
   >
 {
 public:
@@ -50,6 +52,8 @@ public:
   Prevent_deref<I>
   , I // base
   , I // value
+  , boost::use_default
+  , I
   > Base;
   typedef typename Base::reference reference;
   typedef typename std::pair<I, I> range;