fix void_t for older compilers

include/nlohmann/detail/meta/void_t.hpp

@@ -4,7 +4,10 @@ namespace nlohmann
 {
 namespace detail
 {
-template <typename...>
-using void_t = void;
+template <typename ...Ts> struct make_void
+{
+    using type = void;
+};
+template <typename ...Ts> using void_t = typename make_void<Ts...>::type;
 }
 }