Build errors using Xcode 4.3.2

[Paul J. Lucas]

src/util/string/rstring.h:1956:12: error: 'append_safe' is a private member of 'zorba::rstring<zorba::rstring_classes::rep<zorba::atomic_int, std::char_traits<char>, std::allocator<char> > >'
    result.append_safe( s1.data(), s1.size(), s2.data(), s2.size() );
           ^