Define wxMEMBER_DEFAULT similarly to the existing wxOVERRIDE

Also define wxHAS_MEMBER_DEFAULT allowing to check if '= default' is supported
(compilers that define c++11 and VS2015 and later).

include/wx/defs.h

@@ -285,6 +285,14 @@ typedef short int WXTYPE;
     #define wxOVERRIDE
 #endif /*  HAVE_OVERRIDE */
 
+/* same for defaulted member function keyword */
+#if __cplusplus >= 201103L || wxCHECK_VISUALC_VERSION(14)
+    #define wxHAS_MEMBER_DEFAULT
+    #define wxMEMBER_DEFAULT = default
+#else
+    #define wxMEMBER_DEFAULT
+#endif
+
 /*
     Support for nullptr is available since MSVS 2010, even though it doesn't
     define __cplusplus as a C++11 compiler.