PostProcess.hs: ensure that pandoc has happy and alex available
PostProcess.hs: ensure that pandoc has happy and alex available

dmalikov/cabal2nix,jb55/cabal2nix,gridaphobe/cabal2nix,psibi/cabal2nix,spencerjanssen/cabal2nix,Fuuzetsu/cabal2nix,bennofs/cabal2nix