The two functions mono_class_is_nullable and mono_class_get_nullable_param are
private mono symbols, which means we can't call them when using Xamarin.Mac
with libmono from a dynamic library.
Implement a fallback for this case, where we call a managed method when these
functions are not available (and restrict this workaround to Xamarin.Mac only,
since it's not needed for Xamarin.iOS).
Rolf Bjarne Kvinge