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.

TODO

@@ -1,8 +1,4 @@
 * Soon
-** POSIX updates
-See the recent changes about function prototypes in POSIX Yacc.  Implement
-them.
-
 ** scan-code
 The default case is scanning char-per-char.