python3Packages.cffi: use build-system/dependencies

        --replace '/usr/include/libffi' '${lib.getDev libffi}/include'

    '';

    nativeBuildInputs = [

      pkg-config

      setuptools

    ];

    nativeBuildInputs = [ pkg-config ];

    build-system = [ setuptools ];

    buildInputs = [ libffi ];

    propagatedBuildInputs = [ pycparser ];

    dependencies = [ pycparser ];

    # The tests use -Werror but with python3.6 clang detects some unreachable code.

    env.NIX_CFLAGS_COMPILE = lib.optionalString stdenv.cc.isClang "-Wno-unused-command-line-argument -Wno-unreachable-code -Wno-c++11-narrowing";