mpich: fix build on darwin

{ stdenv, lib, fetchurl, perl, gfortran

, openssh, hwloc, python3

, darwin

# either libfabric or ucx work for ch4backend on linux. On darwin, neither of

# these libraries currently build so this argument is ignored on Darwin.

, ch4backend

  nativeBuildInputs = [ gfortran python3 ];

  buildInputs = [ perl openssh hwloc ]

    ++ lib.optional (!stdenv.isDarwin) ch4backend

    ++ lib.optional pmixSupport pmix;

    ++ lib.optional pmixSupport pmix

    ++ lib.optional stdenv.isDarwin darwin.apple_sdk.frameworks.Foundation;

  doCheck = true;

  # test_double_serializer.test fails on darwin

  doCheck = !stdenv.isDarwin;

  preFixup = ''

    # Ensure the default compilers are the ones mpich was built with