Changes: update
v0.32

 - Fix memory leak in Callback objects (GH#46) (thanks, @plicease!)
 - Use PkgConfig when building, if it's available (GH#48) (thanks, @plicease!)
 - Do not require Alien::MSYS if system libffi is available
   (GH#47) (GH#49) (thanks, @plicease!)