Make the xdg dependency optional

We now default to using `~/.cache/waybar-current-task.json` if the `xdg` package is not present.

org-clock-waybar.el

@@ -38,7 +38,7 @@
 
 ;;; Code:
 
-(require 'xdg)
+(require 'xdg nil t)
 (require 'json)
 (require 'org-clock)
 
@@ -47,7 +47,9 @@
   :group 'emacs)
 
 (defcustom org-clock-waybar-filename
-  (expand-file-name "waybar-current-task.json" (xdg-cache-home))
+  (if (fboundp 'xdg-cache-home)
+      (expand-file-name "waybar-current-task.json" (xdg-cache-home))
+    (expand-file-name "~/.cache/waybar-current-task.json"))
   "Name of the file to save task data to."
   :type 'string
   :group 'org-clock-waybar)