Ensure we don't print 'warning:' or 'error:' twice.

Ensure we don't print 'warning:' or 'error:' twice.

* Makefile.am (sc_ensure_testsuite_has_run): New helper rule.
(sc_tests_logs_duplicate_prefixes): New rule, to check for
wrong-looking warning and error messages in the test suite logs.
* aclocal.in (scan_configure_dep, scan_file): Remove duplicate
`warning: ' prefix.
* tests/vars3.test: Rename variable to avoid false positive.

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