an unsupported feature as it has now been removed from Xcode 4.3.2, 
causing lib tool to complain about an unrecognised flag.

I considered removing this entirely but have decided to keep it around 
as an option for the time being. You can get the previous behaviour by
adding set_arc_compatibility_flag! to your Podfile.

I'd like to get rid of this entirely eventually. Re-closes #142 unless
anybody has any objections.