vc6 workaround

[SVN r20906]

include/boost/iterator/detail/minimum_category.hpp

@@ -58,9 +58,15 @@ template <>
 struct minimum_category_impl<false,false>
 {
     template <class T1, class T2> struct apply
+# if BOOST_WORKAROUND(BOOST_MSVC, == 1200)
+    {
+        typedef void type;
+    };
+# else 
     : error_not_related_by_convertibility<T1,T2>
     {
     };
+# endif 
 };
 
 template <class T1 = mpl::_1, class T2 = mpl::_2>