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>