parallel-tests: per-extension test driver: <EXT>_LOG_COMPILER.

parallel-tests: per-extension test driver: <EXT>_LOG_COMPILER.

For test files with extension <ext>, introduce the internal
variable <EXT>_LOG_COMPILE, which expands to
$(<EXT>_LOG_COMPILER) $(AM_<EXT>_LOG_FLAGS) $(<EXT>_LOG_FLAGS).
Turn also the lib/Automake/tests testsuite over to the new
test driver.
* doc/automake.texi (Tests): Document `EXT_LOG_COMPILER' and
`EXT_LOG_FLAGS'.
* lib/am/check2.am: Insert `%COMPILE%' right before test.
* automake.in (handle_tests): Substitute `COMPILE' for check2,
empty for tests without extension, and `$(ext_LOG_COMPILE)' for
extension `ext'.  In the latter case, define it from the public
components.
* configure.ac (AM_INIT_AUTOMAKE): Use `parallel-test' globally.
* tests/Makefile.am (AUTOMAKE_OPTIONS): Remove, not needed here
any more.
* lib/Automake/tests/Makefile.am (TESTS_ENVIRONMENT): Split ...
(PL_LOG_COMPILER, PL_LOG_FLAGS): ... into these new variables.
(TESTS_EXTENSIONS): New variable, initialize to `.pl'.
* tests/parallel-tests7.test: New test.
* tests/Makefile.am: Update.
Suggestion by Akim Demaille.

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