maint: extend the always_defined_macros syntax-check

* cfg.mk (gl_generated_headers_): Define.
(headers_with_interesting_macro_defs): Remove headers covered
by the above.
(.re-defmac): Extract symbol names from many more files.
(sc_always_defined_macros): Use VC_LIST_EXCEPT, not VC_LIST, so
that we can use the usual exception mechanism.
Test for $(gnulib_dir), not system.h.
* .x-sc_always_defined_macros: New file.  Exempt src/seq.c.
* Makefile.am (syntax_check_exceptions): Add it here.

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