Fixed https://github.com/mapbox/windows-builds/issues/88

(inheriting constructors not working with msvc-14.0 2015)

include/mapnik/json/generic_json.hpp

@@ -57,7 +57,16 @@ using json_value_base = mapnik::util::variant<value_null,
                                               mapnik::util::recursive_wrapper<json_object> >;
 struct json_value : json_value_base
 {
+
+#ifdef _WINDOWS
+    json_value() = default;
+    template <typename T>
+    json_value(T && val)
+        : json_value_base(std::forward<T>(val)) {}
+#else
+    // MSVC 2015  inheriting constructors is not working in this context (support is apparently planned)
     using json_value_base::json_value_base;
+#endif
 };
 
 using uchar = std::uint32_t; // a unicode code point