tests: prefer 'configure.ac' over 'configure.in'

tests: prefer 'configure.ac' over 'configure.in'

With this change, our testsuite now uses 'configure.ac' as the
name for the typical autoconf input, instead of the obsolescent
'configure.in' (which has been deprecated for several years, at
least since autoconf 2.50).

Most of the test cases changed by this commit have been
automatically modified with this sed command (using GNU
sed):

  sed -i 's/\<configure\(\\\?\)\.in\>/configure\1.ac/g'

* syntax-checks.mk (sc_tests_no_configure_in): New syntax check.
(syntax_check_rules): Add it.
* tests/defs: Create stub for autoconf input in 'configure.ac',
not in 'configure.in'.  Adjust comments.
($AUTOCONF, $AUTOHEADER, $AUTORECONF): Remove workaround for the
infamous debian autoconf wrapper: we don't need such workaround
anymore now that we name our autoconf input as 'configure.ac'.
For more information, see commit v1.11-564-g63da492 "test defs:
hack to support autoconf-wrapper programs" of 16-12-2011.
* tests/README: Use 'configure.ac', not 'configure.in'.
* Many many tests: Likewise.
* tests/backcompat3.test: Adjust to avoid spurious failures.
* tests/backcompat5.test: Likewise.
* tests/missing6.test: Likewise.
* tests/backcompat6.test: Likewise, and extend a bit since we
are at it.