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 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.
* .x-sc_file_system: Remove file.
* .x-sc_prohibit_tab_based_indentation: Likewise.
* .x-sc_require_config_h: Likewise.
* .x-sc_require_config_h_first: Likewise.
* cfg.mk: Define variables to exempt the same files.