Updated Optional with fix for GCC 4.8 on OSX.

Thanks to Stefan Wasilewski for reporting.

src/MagnumExternal/Optional/optional.hpp

@@ -151,7 +151,7 @@ template <class T> inline constexpr typename std::remove_reference<T>::type&& co
     __assert(expr, line, file); // WHY.
   # elif defined __ANDROID__
     __assert(file, line, expr);
-  # elif defined __clang__ || defined __GNU_LIBRARY__
+  # elif defined __clang__ || defined __GNU_LIBRARY__ || (defined __GNUC__ && defined __APPLE__)
     __assert(expr, file, line);
   # elif defined __GNUC__
     _assert(expr, file, line);