avoid C style casts

[SVN r53672]

include/boost/smart_ptr/detail/sp_convertible.hpp

@@ -45,7 +45,7 @@ template< class Y, class T > struct sp_convertible
     static yes f( T* );
     static no  f( ... );
 
-    enum _vt { value = sizeof( f( (Y*)0 ) ) == sizeof(yes) };
+    enum _vt { value = sizeof( f( static_cast<Y*>(0) ) ) == sizeof(yes) };
 };
 
 struct sp_empty