build: avoid possibly-replaced fprintf in liby-source, yyerror.c

build: avoid possibly-replaced fprintf in liby-source, yyerror.c

* lib/yyerror.c (yyerror): Use fputs and fputc rather than fprintf
with a mere "%s\n" format.  Always return 0 now, on the assumption
that the return value was never used anyway.
Don't include <config.h> after all.  This avoids a problem
reported by Thiru Ramakrishnan in
http://lists.gnu.org/archive/html/help-bison/2011-11/msg00000.html
* cfg.mk: Exempt lib/yyerror.c from the sc_require_config_h_first test.
* THANKS: Update.
(cherry picked from commit fb9eea88856d73a5f25295a52f6d4df7fabc565b)