configure.in: remove --with-mobile

It doesn't actually do anything. Was quite
misleading.
build: use MOBILE_DYNAMIC instead of MONODROID

Some conditional compilation shouldn't really
be bound to MONODROID, instead add a new define
that can be reused, MOBILE_DYNAMIC.
profiles: define xammac (Xamarin.Mac) profile

Based on the Mobile profile and modeled after
the MonoDroid profile.