* configure.ac.in, modules/codec/Modules.am, modules/codec/tarkin.c: added
   an experimental tarkin video decoder. You will need to specify the tarkin
   tree you want to use with --with-tarkin-tree=.

   Warning: Tarkin is still in very early development stage, so this plugin isn't
   actually usefull unless you intend to do some development on tarkin itself.