Make shared_from_this and weak_from_this private in enable_shared_from. Fixes #75.

test/shared_from_fail.cpp

@@ -0,0 +1,16 @@
+// Copyright 2020 Peter Dimov
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/shared_ptr.hpp>
+#include <boost/smart_ptr/enable_shared_from.hpp>
+
+struct X: public boost::enable_shared_from
+{
+};
+
+int main()
+{
+    boost::shared_ptr<X> px( new X );
+    px->shared_from_this();
+}