yacc: comply with recent POSIX updates: declare yyerror and yylex

In POSIX Yacc mode, declare yyerror and yylex unless already #defined,
or if YYERROR_IS_DECLARED/YYLEX_IS_DECLARED are defined (for
consistency with Bison's YYSTYPE_IS_DECLARED/YYLTYPE_IS_DECLARED).
See <https://austingroupbugs.net/view.php?id=1388#c5220>.

* data/skeletons/c.m4 (b4_function_declare): Resurect.
(b4_lex_formals): Since we will possibly expose this prototype
in the header, take the prefix into account.
* data/skeletons/yacc.c (b4_declare_yyerror_and_yylex): New.
(b4_shared_declarations): Use it.

* tests/local.at (AT_YACC_IF): New.
When in Yacc mode, set the `yacc` Autotest keyword.
(AT_YYERROR_DECLARE(c)): Don't declare in Yacc mode,
to avoid clashes (since this signature is static).
(AT_YYERROR_DEFINE(c)): Don't define as static in Yacc mode.
* tests/regression.at (Early token definitions with --yacc): Specify
that we are in Yacc mode.

NEWS

@@ -9,6 +9,14 @@ GNU Bison NEWS
   now generates a *.gv file by default, instead of *.dot.  A transition
   started in Bison 3.4.
 
+  To comply with the latest POSIX standard, in Yacc compatibility mode
+  (options `-y`/`--yacc`) Bison now generates prototypes for yyerror and
+  yylex.  In some situations, this is breaking compatibility: if the user
+  has already declared these functions but with some differences (e.g., to
+  declare them as static, or to use specific attributes), the generated
+  parser will fail to compile.  To disable these prototypes, #define yyerror
+  (to `yyerror`), and likewise for yylex.
+
 ** Deprecated features
 
   Support for the YYPRINT macro is removed. It worked only with yacc.c and