Revert "SmartPtr: Remove obsolete MSVC version checks."

This reverts commit 7d1c527ac0.

include/boost/smart_ptr/weak_ptr.hpp

@@ -136,6 +136,8 @@ public:
         boost::detail::sp_assert_convertible< Y, T >();
     }
 
+#if !defined(BOOST_MSVC) || (BOOST_MSVC >= 1300)
+
     template<class Y>
     weak_ptr & operator=( weak_ptr<Y> const & r ) BOOST_NOEXCEPT
     {
@@ -169,6 +171,8 @@ public:
         return *this;
     }
 
+#endif
+
     shared_ptr<T> lock() const BOOST_NOEXCEPT
     {
         return shared_ptr<T>( *this, boost::detail::sp_nothrow_tag() );