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.

tests/README.md

@@ -27,6 +27,7 @@ synonyms.
 - report: for automaton dumps
 - %union
 - variant
+- yacc: POSIX yacc (%yacc)
 
 # Calculator
 The grammar features several special directives:

tests/local.at

@@ -275,6 +275,8 @@ m4_pushdef([AT_MULTISTART_IF],
 [m4_bmatch([$3], [%start [_a-zA-Z]+ [_a-zA-Z]+], [$1], [$2])])
 m4_pushdef([AT_PARAM_IF],
 [m4_bmatch([$3], [%parse-param], [$1], [$2])])
+m4_pushdef([AT_YACC_IF],
+[m4_bmatch([$3], [%yacc], [$1], [$2])])
 
 # Comma-terminated list of formals parse-parameters.
 # E.g., %parse-param { int x } %parse-param {int y} -> "int x, int y, ".
@@ -427,6 +429,7 @@ AT_LOCATION_TYPE_SPAN_IF(
 AT_GLR_IF([AT_KEYWORDS([glr])])
 AT_MULTISTART_IF([AT_KEYWORDS([multistart])])
 AT_PUSH_IF([AT_KEYWORDS([push])])
+AT_YACC_IF([AT_KEYWORDS([yacc])])
 ])# _AT_BISON_OPTION_PUSHDEFS
 
 
@@ -466,6 +469,7 @@ m4_popdef([AT_VALUE_UNION_IF])
 m4_popdef([AT_PUSH_IF])
 m4_popdef([AT_PURE_IF])
 m4_popdef([AT_PARSER_CLASS])
+m4_popdef([AT_YACC_IF])
 m4_popdef([AT_PARAM_IF])
 m4_popdef([AT_MULTISTART_IF])
 m4_popdef([AT_LEXPARAM_IF])
@@ -683,7 +687,7 @@ m4_define([AT_YYERROR_DECLARE_EXTERN(c)],
 m4_define([AT_YYERROR_DECLARE(c)],
 [[#include <stdio.h>
 ]AT_LOCATION_PRINT_DECLARE[
-static ]AT_YYERROR_DECLARE_EXTERN])
+]AT_YACC_IF([], [[static ]AT_YYERROR_DECLARE_EXTERN])])
 
 
 # "%define parse.error custom" uses a different format, easy to check.
@@ -729,7 +733,7 @@ yyreport_syntax_error (const yypcontext_t *ctx]AT_PARAM_IF([, AT_PARSE_PARAMS])[
 ]])[
 
 /* A C error reporting function.  */
-static
+]AT_YACC_IF([], [static])[
 ]AT_YYERROR_PROTOTYPE[
 {]m4_bpatsubst(m4_defn([AT_PARSE_PARAMS]),
               [[^,]+[^A-Za-z_0-9]\([A-Za-z_][A-Za-z_0-9]*\),* *], [

tests/regression.at

@@ -87,7 +87,7 @@ AT_SETUP([Early token definitions with --yacc])
 # Found in GCJ: they expect the tokens to be defined before the user
 # prologue, so that they can use the token definitions in it.
 
-AT_BISON_OPTION_PUSHDEFS
+AT_BISON_OPTION_PUSHDEFS([%yacc])
 
 # Not AT_DATA_GRAMMAR, which uses %code, which is not supported by Yacc.
 AT_DATA([input.y],
@@ -112,7 +112,7 @@ exp: MY_TOKEN;
 ]])
 AT_BISON_OPTION_POPDEFS
 
-AT_BISON_CHECK([-y -o input.c input.y])
+AT_BISON_CHECK([--yacc -o input.c input.y])
 AT_COMPILE([input.o])
 
 AT_CLEANUP