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.

tests/calc.at

@@ -268,7 +268,7 @@ read_integer (]AT_YYLEX_FORMALS[)
 
 
 m4_define([_AT_DATA_CALC_Y(c)],
-[AT_DATA_GRAMMAR([calc.y],
+[AT_DATA_GRAMMAR([calc.y.tmp],
 [[/* Infix notation calculator--calc */
 ]$4[
 %code requires
@@ -493,6 +493,18 @@ location_print (FILE *o, Span s)
 [AT_CALC_YYLEX
 AT_CALC_MAIN])])
 
+# Remove the generated prototypes.
+AT_CHECK(
+  [AT_YACC_IF([[
+    if "$POSIXLY_CORRECT_IS_EXPORTED"; then
+      sed -e '/\/\* !POSIX \*\//d' calc.y.tmp >calc.y
+    else
+      mv calc.y.tmp calc.y
+    fi
+  ]],
+  [[mv calc.y.tmp calc.y]])
+])
+
 AT_HEADER_IF([AT_DATA_SOURCE([[calc-lex.]AT_LANG_EXT],
 [[#include "calc.]AT_LANG_HDR["