maint: stop using .x-sc_* files to list syntax-check exemptions

maint: stop using .x-sc_* files to list syntax-check exemptions

Instead, use the brand new mechanism with which you merely use a
variable (derived from the rule name) defined in cfg.mk to an ERE
matching the exempted file names.
* gnulib: Update to latest, to get maint.mk that implements this.
* Makefile.am (syntax_check_exceptions): Remove variable.
(EXTRA_DIST): Remove use of the variable.
* cfg.mk (sc_x_sc_dist_check): Remove rule, no longer useful.
(exclude_file_name_regexp--sc_space_tab): Define variable.
(exclude_file_name_regexp--sc_bindtextdomain): Likewise.
(exclude_file_name_regexp--sc_unmarked_diagnostics): Likewise.
(exclude_file_name_regexp--sc_error_message_uppercase): Likewise.
(exclude_file_name_regexp--sc_trailing_blank): Likewise.
(exclude_file_name_regexp--sc_system_h_headers): Likewise.
(exclude_file_name_regexp--sc_require_config_h_first): Likewise.
(exclude_file_name_regexp--sc_require_config_h): Likewise.
(exclude_file_name_regexp--sc_po_check): Likewise.
(exclude_file_name_regexp--sc_prohibit_always-defined_macros): Likewise.
(exclude_file_name_regexp--sc_prohibit_empty_lines_at_EOF): Likewise.
(exclude_file_name_regexp--sc_program_name): Likewise.
(exclude_file_name_regexp--sc_file_system): Likewise.
(exclude_file_name_regexp--sc_prohibit_always_true_header_tests):
Likewise.
(exclude_file_name_regexp--sc_prohibit_fail_0): Likewise.
(exclude_file_name_regexp--sc_prohibit_atoi_atof): Likewise.
(exclude_file_name_regexp--sc_prohibit_tab_based_indentation): Likewise.
(exclude_file_name_regexp--sc_prohibit_stat_st_blocks): Likewise.
* configure.ac [whether localtime caches TZ]: Use return 0/1, not
exit (0/1) to avoid triggering a sc_prohibit_magic_number_exit failure.
* .x-sc_GPL_version: Remove file.
* .x-sc_bindtextdomain: Likewise.
* .x-sc_error_message_uppercase: Likewise.
* .x-sc_file_system: Likewise.
* .x-sc_obsolete_symbols: Likewise.
* .x-sc_po_check: Likewise.
* .x-sc_program_name: Likewise.
* .x-sc_prohibit_always-defined_macros: Likewise.
* .x-sc_prohibit_always_true_header_tests: Likewise.
* .x-sc_prohibit_atoi_atof: Likewise.
* .x-sc_prohibit_empty_lines_at_EOF: Likewise.
* .x-sc_prohibit_fail_0: Likewise.
* .x-sc_prohibit_magic_number_exit: Likewise.
* .x-sc_prohibit_stat_st_blocks: Likewise.
* .x-sc_prohibit_strcmp: Likewise.
* .x-sc_prohibit_tab_based_indentation: Likewise.
* .x-sc_require_config_h: Likewise.
* .x-sc_require_config_h_first: Likewise.
* .x-sc_space_tab (config): Likewise.
* .x-sc_sun_os_names: Likewise.
* .x-sc_system_h_headers: Likewise.
* .x-sc_trailing_blank: Likewise.
* .x-sc_unmarked_diagnostics: Likewise.
* .x-sc_useless_cpp_parens: Likewise.