yacc: declare yyerror/yylex only when POSIXLY_CORRECT

The recent changes to comply with POSIX are breaking Automake's test
suite.
Reported by Kiyoshi Kanazawa.
<https://lists.gnu.org/r/bug-bison/2021-09/msg00005.html>

To limit the impact of POSIX changes, bind them to $POSIXLY_CORRECT.
Suggested by Karl Berry.
<https://lists.gnu.org/r/bug-bison/2021-09/msg00009.html>

The existing `maintainer-check-posix` Make target checks these
changes.

* src/getargs.h, src/getargs.c (set_yacc): New.
Use it.
* data/skeletons/bison.m4 (b4_posix_if): New.
* data/skeletons/yacc.c (b4_declare_yyerror_and_yylex): Use it.
* doc/bison.texi, tests/local.at: Adjust.

src/getargs.h

@@ -140,6 +140,7 @@ void getargs (int argc, char *argv[]);
 /* Used by parse-gram.y.  */
 void language_argmatch (char const *arg, int prio, location loc);
 void skeleton_arg (const char *arg, int prio, location loc);
+void set_yacc (location loc);
 
 /** In the string \c s, replace all characters \c from by \c to.  */
 void tr (char *s, char from, char to);