maint.mk: new syntax check for HAVE_DECL checks

maint.mk: new syntax check for HAVE_DECL checks

Based on Karel's report, we might as well enforce a syntax check
to help avoid future problems.
https://lists.gnu.org/archive/html/coreutils/2012-06/msg00037.html

Exempt a few files that still get picked up in the wild rather
than as a full gnulib module with m4 files.

* top/maint.mk (sc_prohibit_defined_have_decl_tests): New rule.
* cfg.mk
(exclude_file_name_regexp--sc_prohibit_defined_have_decl_tests):
Exempt some false positives.
Based on a report by Karel Zak.

Signed-off-by: Eric Blake <eblake@redhat.com>