external: updated Optional with Native Client support.

external/Optional/optional.hpp

@@ -147,6 +147,8 @@ template <class T> inline constexpr typename std::remove_reference<T>::type&& co
   {
   # if defined(EMSCRIPTEN) && EMSCRIPTEN
     __assert_fail(expr, file, line, "");
+  # elif defined __native_client__
+    __assert(expr, line, file); // WHY.
   # elif defined __clang__ || defined __GNU_LIBRARY__
     __assert(expr, file, line);
   # elif defined __GNUC__