Fix: prevented the binding illegal temporary to optional<const int&>

Older MSVC versions add illegal temporary when you want to assign from const integral value.

doc/html/boost_optional/relnotes.html

@@ -43,6 +43,10 @@
 <li class="listitem">
                 some bugs connected to copying optional references are gone,
               </li>
+<li class="listitem">
+                all run-time bugs caused by incorrect reference binding on some compilers
+                are now turned into compile-time errors,
+              </li>
 <li class="listitem">
                 you can swap optional references: it is like swapping pointers: shalow,
                 underlying objects are not affected,