Avoid 'reference to reference' error in C++03.

Some (strict) C++03 compilers (e.g. `gcc -std=c++03`) reject
  'reference-to-reference' in the template and typedef which described
  in CWG DR106 [1].

  In such situations, iterator_facade rejects reference type as a value
  type and some special iterators will become ill-formed:
  the test libs/range/test/join.hpp might be descriptive.

  [1] http://www.open-std.org/Jtc1/sc22/wg21/docs/cwg_defects.html#106

Signed-off-by: Kohei Takahashi <flast@flast.jp>

include/boost/iterator/iterator_facade.hpp

@@ -22,6 +22,7 @@
 #include <boost/type_traits/is_same.hpp>
 #include <boost/type_traits/add_const.hpp>
 #include <boost/type_traits/add_pointer.hpp>
+#include <boost/type_traits/add_lvalue_reference.hpp>
 #include <boost/type_traits/remove_const.hpp>
 #include <boost/type_traits/remove_reference.hpp>
 #include <boost/type_traits/is_convertible.hpp>
@@ -284,7 +285,15 @@ namespace iterators {
       : mpl::eval_if<
             mpl::and_<
                 // A proxy is only needed for readable iterators
-                is_convertible<Reference,Value const&>
+                is_convertible<
+                    Reference
+                    // Use add_lvalue_reference to form `reference to Value` due to
+                    // some (strict) C++03 compilers (e.g. `gcc -std=c++03`) reject
+                    // 'reference-to-reference' in the template which described in CWG
+                    // DR106.
+                    // http://www.open-std.org/Jtc1/sc22/wg21/docs/cwg_defects.html#106
+                  , typename add_lvalue_reference<Value const>::type
+                >
 
                 // No multipass iterator can have values that disappear
                 // before positions can be re-visited