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 fb9eea8885)

lib/yyerror.c

@@ -17,7 +17,6 @@
    You should have received a copy of the GNU General Public License
    along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
 
-#include <config.h>
 #include <stdio.h>
 
 int yyerror (char const *);
@@ -25,5 +24,7 @@ int yyerror (char const *);
 int
 yyerror (char const *message)
 {
-  return fprintf (stderr, "%s\n", message);
+  fputs (message, stderr);
+  fputc ('\n', stderr);
+  return 0;
 }