Add new macro for standard library header inclusion

Newer standard library headers should only be included when the compiler
is targetting that standard, otherwise some compilers (like MSVC) will
warn that you are using a newer C++ include on an older version.

tests/strings/stdstrings.cpp

@@ -691,4 +691,4 @@ TEST_CASE("StdString::View", "[stdstring]")
 
     CHECK( "" == wxString::FromUTF8(strViewInvalidUTF) );
 }
-#endif
+#endif // wxHAS_STD_STRING_VIEW