fvwm: add gestures option to use libstroke

pkgs/applications/window-managers/fvwm/default.nix

···
-{ stdenv, fetchurl, pkgconfig
+{ gestures ? false
+, stdenv, fetchurl, pkgconfig
 , cairo, fontconfig, freetype, libXft, libXcursor, libXinerama
 , libXpm, librsvg, libpng, fribidi, perl
+, libstroke ? null
 }:
+
+assert gestures -> libstroke != null;
 
 stdenv.mkDerivation rec {
   name = "fvwm-2.6.5";
···
     pkgconfig cairo fontconfig freetype
     libXft libXcursor libXinerama libXpm
     librsvg libpng fribidi perl
-  ];
+  ] ++ stdenv.lib.optional gestures libstroke;
 
   meta = {
     homepage = "http://fvwm.org";