gtask: Only override g_task_set_source_tag() for GLib ≥ 2.60

g_task_set_name() was added in GLib 2.60, so only use it in the
overridden definition of g_task_set_source_tag() if the user has said
that they require GLib ≥ 2.60.

This is a follow up to commit b08bd04a.
Signed-off-by: Philip Withnall <withnall@endlessm.com>

gio/gtask.h

@@ -79,12 +79,14 @@ void          g_task_set_name              (GTask               *task,
                                             const gchar         *name);
 
 /* Macro wrapper to set the task name when setting the source tag. */
+#if GLIB_VERSION_MIN_REQUIRED >= GLIB_VERSION_2_60
 #define g_task_set_source_tag(task, tag) G_STMT_START { \
   GTask *_task = (task); \
   (g_task_set_source_tag) (_task, tag); \
   if (g_task_get_name (_task) == NULL) \
     g_task_set_name (_task, G_STRINGIFY (tag)); \
 } G_STMT_END
+#endif
 
 GLIB_AVAILABLE_IN_2_36
 gpointer      g_task_get_source_object     (GTask               *task);