testsuite: log version of M4 and Perl

testsuite: log version of M4 and Perl

Jannick reported problems on OSX for which the most plausible
explanation is that the system-provided M4 is emitting error messages
with different line numbers than we expect, perhaps because Apple
froze their copy of GNU M4 to the last GPLv2 release.  To test this
hypothesis, add $PERL and $M4 to AT_TESTED in our testsuite.

* tests/atlocal.in: Also set $M4 from configure.
* tests/local.at: Add AT_TESTED([$PERL $M4]).