update post-processing for liquid-fixpoint and liquidhaskell.
update post-processing for liquid-fixpoint and liquidhaskell.

liquid-fixpoint needs the -fbuild-external flag to force building the
ocaml binary locally instead of using a precompiled binary (not
guaranteed to work in nixos AFAIK)

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