rPackages.unigd: fix build on darwin (#379402)

    shrinkTVP = [ pkgs.gsl ];

    sbrl = with pkgs; [ gsl gmp.dev ];

    surveyvoi = with pkgs; [ gmp.dev mpfr.dev ];

    unigd = with pkgs; [ cairo.dev libpng.dev ];

    unigd = with pkgs; [ cairo.dev libpng.dev ] ++ lib.optionals stdenv.hostPlatform.isDarwin [ expat xorg.libXdmcp ];

    HilbertVisGUI = [ pkgs.gtkmm2.dev ];

    textshaping = with pkgs; [ harfbuzz.dev freetype.dev fribidi libpng ];

    DropletUtils = [ pkgs.zlib.dev ];