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