external: updated Optional with Android support.

src/MagnumExternal/Optional/optional.hpp

@@ -149,6 +149,8 @@ template <class T> inline constexpr typename std::remove_reference<T>::type&& co
     __assert_fail(expr, file, line, "");
   # elif defined __native_client__
     __assert(expr, line, file); // WHY.
+  # elif defined __ANDROID__
+    __assert(file, line, expr);
   # elif defined __clang__ || defined __GNU_LIBRARY__
     __assert(expr, file, line);
   # elif defined __GNUC__