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.

cfg.mk

@@ -55,4 +55,5 @@ update-copyright-env = \
   UPDATE_COPYRIGHT_FORCE=1 UPDATE_COPYRIGHT_USE_INTERVALS=1
 
 exclude_file_name_regexp--sc_space_tab = ^tests/(input|c\+\+)\.at$$
-exclude_file_name_regexp--sc_require_config_h_first = ^data/(glr|yacc)\.c$$
+exclude_file_name_regexp--sc_require_config_h_first = \
+  ^(lib/yyerror|data/(glr|yacc))\.c$$