C_IN macro was added years ago in bcbaf1be, using same value as the
internal code of Android with the reasonning that "some parts of the API
used by the resolver objects is not public in the Android NDK (yet)".
Well since then things are changed, since it is definitely available (at
least on the API 22 of Android which I am using) in the public header
arpa/nameser_compat.h.
Let's just add a #ifndef to handle both cases when you build with an
older or recent API.