libspnav: fix dylib install name on darwin

pkgs/development/libraries/libspnav/default.nix

···
-{ stdenv, lib, fetchFromGitHub, libX11}:
+{ stdenv, lib, fetchFromGitHub, libX11, fixDarwinDylibNames }:
 
 stdenv.mkDerivation rec {
   version = "0.2.3";
···
     sha256 = "098h1jhlj87axpza5zgy58prp0zn94wyrbch6x0s7q4mzh7dc8ba";
   };
 
+  nativeBuildInputs = lib.optional stdenv.isDarwin fixDarwinDylibNames;
   buildInputs = [ libX11 ];
 
   configureFlags = [ "--disable-debug"];