gcal: depend on gettext on Darwin

Without gettext the gcal binary crashes on macOS:

$ ./src/gcal
Illegal instruction: 4

pkgs/applications/misc/gcal/default.nix

···
-{ lib, stdenv, fetchurl, ncurses, fetchpatch }:
+{ lib, stdenv, fetchurl, ncurses, gettext, fetchpatch }:
 
 stdenv.mkDerivation rec {
   pname = "gcal";
···
 
   enableParallelBuilding = true;
 
-  buildInputs = [ ncurses ];
+  buildInputs = [ ncurses ] ++ lib.optional stdenv.isDarwin gettext;
 
   meta = {
     description = "Program for calculating and printing calendars";