tests: detect missing perl at configure runtime

tests: detect missing perl at configure runtime

* configure.ac (AM_CONDITIONAL): Set the conditional 'HAVE_PERL' to
true if the configure-time checks (as run by gl_PERL) have been able
to find a working perl.
* tests/no-perl: New script, report a diagnostic about "missing perl"
and exit with status 77.
* tests/Makefile.am (EXTRA_DIST): Distribute it.
(TESTSUITE_PERL): New, define to '$(PERL)' if a perl interpreter has
been found at configure time (i.e., if the 'HAVE_PERL' automake
conditional is true), and to '$(srcdir)/no-perl' otherwise.
(LOG_COMPILER): Use $(TESTSUITE_PERL) instead of $(PERL).
(XPL_LOG_COMPILER): Likewise.
* tests/shell-or-perl: Simplify: no need to actually check whether
perl is working.