threaded-am: better serialization for required config files With this change, we make serialization/de-serialization of required config files installs more granular, and in the process fix the bug introduced by commit `v1.11-1219-g326ecba'. * automake.in ($required_conf_file_queue): Move its declaration earlier. (require_file_internal): Add a new argument telling whether the function should act immediately or queue its action for the master thread to handle. (queue_required_conf_file): Renamed ... (queue_required_file_check_or_copy): ... to this. (require_queued_conf_file): Renamed ... (require_queued_file_check_or_copy): ... to this, and make it call `required_file_check_or_copy' instead of `require_file_internal'. (require_conf_file, handle_makefiles_threaded): Adjust and simplify accordingly. * tests/Makefile.am (XFAIL_TESTS): Remove `parallel-am.test'.