tests: factor the declaration/definition of yyerror and yylex.

* tests/local.at (AT_YYERROR_DECLARE, AT_YYERROR_DECLARE_EXTERN)
(AT_YYERROR_DEFINE, AT_YYLEX_DECLARE, AT_YYLEX_DECLARE_EXTERN)
(AT_YYLEX_DEFINE): New.
Must be used inside AT_BISON_OPTION_PUSHDEFS/POPDEFS pair.
* tests/actions.at, tests/conflicts.at, tests/glr-regression.at,
* tests/headers.at, tests/input.at, tests/named-refs.at,
* tests/regression.at, tests/skeletons.at, tests/synclines.at,
* tests/torture.at: Use them.

tests/skeletons.at

@@ -83,10 +83,11 @@ AT_CLEANUP
 
 AT_SETUP([[Installed skeleton file names]])
 
+AT_BISON_OPTION_PUSHDEFS
 m4_pushdef([AT_GRAM],
 [[%{
   #include <stdio.h>
-  void yyerror (char const *msg);
+  ]AT_YYERROR_DECLARE[
   int yylex (void);
 %}
 
@@ -99,12 +100,7 @@ start: ;
 
 %%
 
-void
-yyerror (char const *msg)
-{
-  fprintf (stderr, "%s\n", msg);
-}
-
+]AT_YYERROR_DEFINE[
 int
 yylex (void)
 {
@@ -139,6 +135,7 @@ AT_PARSER_CHECK([[./input-gram]], [[1]], [],
 
 m4_popdef([AT_GRAM])
 
+AT_BISON_OPTION_POPDEFS
 AT_CLEANUP