make: serialize strip-po / strip-pot
    
    The strip-po target modifies files in place. This sometimes creates
    conflicts with other make targets when make is run in parallel mode.
    
    * split strip-po into strip-po and strip-pot
    * move strip-po[t] from dependency to explicit, serial execution
    * declare dependencies on POT/POFILES
    * don't run strip on clean
    
    Fixes: https://pagure.io/freeipa/issue/8323
    Signed-off-by: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>