Allow using Bind() with non-public inheritance in C++11 code

Using Bind() with a method of the class deriving from wxEvtHandler
non-publicly used to result in a compile-time error, but at least with C++11
we can detect this case and allow the code to compile.

Closes #17623.

tests/events/evthandler.cpp

@@ -509,3 +509,21 @@ void EvtHandlerTestCase::UnbindFromHandler()
 
     handler.ProcessEvent(e);
 }
+
+// This is a compilation-time-only test: just check that a class inheriting
+// from wxEvtHandler non-publicly can use Bind() with its method, this used to
+// result in compilation errors.
+// Note that this test will work only on C++11 compilers, so we test this only
+// for such compilers.
+#if __cplusplus >= 201103
+class HandlerNonPublic : protected wxEvtHandler
+{
+public:
+    HandlerNonPublic()
+    {
+        Bind(wxEVT_IDLE, &HandlerNonPublic::OnIdle, this);
+    }
+
+    void OnIdle(wxIdleEvent&) { }
+};
+#endif // C++11