maint: move syntax check exemptions into cfg.mk

It's easier to maintain exemptions in a single file than it is
to maintain a bunch of hidden .x files.
* .x-sc_cast_of_argument_to_free: Delete file.
* .x-sc_prohibit_tab_based_indentation: Likewise.
* .x-sc_require_config_h: Likewise.
* .x-sc_require_config_h_first: Likewise.
* Makefile.am (syntax_check_exceptions): Delete rule.
* cfg.mk (exclude_file_name_regexp): Move exemptions here.

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