Parallel test execution: new option `parallel-tests'.

Parallel test execution: new option `parallel-tests'.

* automake.in (handle_tests): Set new conditional PARALLEL_TESTS
when reading check.am.  Handle option `parallel-tests' and all
its semantics.  Define macros TEST_EXTENSIONS, TEST_SUITE_LOG,
TEST_SUITE_HTML, TEST_LOGS, TEST_LOGS_TMP, suffix rules if
applicable, and per-target rules for other tests.  Add all log
files to %clean_files at the `MOSTLY_CLEAN' level.
* lib/Automake/Options.pm (_process_option_list): Accept
`parallel-tests'.
* lib/am/check.am [!PARALLEL_TESTS] (check-TESTS): Move existing
testsuite driver under this new conditional.
[PARALLEL_TESTS]
(am__rst_title, am__rst_section, am__text_box am__sh_e_setup)
(am__check_pre, am__check_post): New internal macros.
($(TEST_SUITE_LOG), check-TESTS, .log.html, check-html): New
rules.
* lib/am/check2.am: New file.
* lib/am/Makefile.am (dist_am_DATA): Add check2.am.
* tests/Makefile.am (AUTOMAKE_OPTIONS): Use `parallel-tests'.
(clean-local): Renamed from distclean-local.
* tests/defs.in: Drop VERBOSE handling, not needed here any more.

Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>