GHC generics are used to derive binary instances for types appearing
in the persistent build config, which requires GHC >= 7.2 and
binary >= 0.7. Unfortunately, GHC < 7.8 ships with binary == 0.5.*.
The missing module is Data.Binary.Generics, which we have copied from
binary-0.7.2.3 to Distribution.Compat.Binary.Generics. To provide
generic implementations for the Binary class, we also have to provide
our own implementation, which is copied from binary-0.7.2.3 to
Distribution.Compat.Binary.Class. The interface required by Cabal is
exported from Distribution.Compat.Binary. This is only done if
bootstrapping Cabal with GHC < 7.8 or if binary >= 0.7 is not available,
otherwise Distribution.Compat.Binary simply re-exports Data.Binary.