parse.lac: implement as %define variable.

LAC = lookahead correction.  See discussion at
<http://lists.gnu.org/archive/html/bison-patches/2009-09/msg00034.html>.
However, one point there must be corrected: because of %nonassoc,
LAC is *not* always redundant for lr.type=canonical-lr.
* data/yacc.c: Accept values of "none" (default) or "full" for
parse.lac.  Accept %define parse.lac.es-capacity to specify
capacity of LAC's temporary exploratory stack.  It defaults to 20
and, for now, will not grow dynamically.
(b4_lac_flag, b4_lac_if): New m4 macros.  Evaluate as true for
parse.lac!=none.
(YYBACKUP): Invoke YY_LAC_DISCARD.
(YY_LAC_ESTABLISH, YY_LAC_DISCARD): New cpp macros that invoke
yy_lac and track when it needs to be invoked
(yy_lac): New function that, given the current stack, determines
whether a token can eventually be shifted.  Return status mimics
yyparse return status.
(yysyntax_error): Change yystate argument to yyssp so stack top
can be passed to yy_lac.  If LAC is requested, build expected
token list by invoking yy_lac for every token instead of just
checking the current state for lookaheads.  Return 2 if yy_lac
exhausts memory.
(yyparse, yypush_parse): Use local variable yy_lac_established and
cpp macros YY_LAC_ESTABLISH and YY_LAC_DISCARD to implement LAC.
Update yysyntax_error invocation.  Add yyexhaustedlab code if LAC
is requested.
* tests/conflicts.at (%nonassoc and eof): Extend to check the
effect of each of -Dlr.type=canonical-lr and -Dparse.lac=full.
(parse.error=verbose and consistent errors): Likewise.
(LAC: %nonassoc requires splitting canonical LR states): New test
group demonstrating how LAC can fix canonical LR.
* tests/input.at (LAC: Errors for %define): New test group.
* tests/regression.at (LAC: Exploratory stack): New test group.
(LAC: Memory exhaustion): New test group.

src/parse-gram.h

@@ -1,4 +1,4 @@
-/* A Bison parser, made by GNU Bison 2.4.537-34db.  */
+/* A Bison parser, made by GNU Bison 2.4.542-10bae.  */
 
 /* Interface for Bison's Yacc-like parsers in C
 
@@ -31,7 +31,7 @@
    This special exception was added by the Free Software Foundation in
    version 2.2 of Bison.  */
 /* "%code requires" blocks.  */
-/* Line 1681 of yacc.c  */
+/* Line 1904 of yacc.c  */
 #line 202 "src/parse-gram.y"
 
 # ifndef PARAM_TYPE
@@ -46,7 +46,7 @@
 # endif
 
 
-/* Line 1681 of yacc.c  */
+/* Line 1904 of yacc.c  */
 #line 51 "src/parse-gram.h"
 
 /* Tokens.  */
@@ -175,7 +175,7 @@
 #if ! defined YYSTYPE && ! defined YYSTYPE_IS_DECLARED
 typedef union YYSTYPE
 {
-/* Line 1681 of yacc.c  */
+/* Line 1904 of yacc.c  */
 #line 88 "src/parse-gram.y"
 
   assoc assoc;
@@ -188,13 +188,13 @@ typedef union YYSTYPE
   uniqstr uniqstr;
   unsigned char character;
 
-/* Line 1681 of yacc.c  */
+/* Line 1904 of yacc.c  */
 #line 226 "src/parse-gram.y"
 
   param_type param;
 
 
-/* Line 1681 of yacc.c  */
+/* Line 1904 of yacc.c  */
 #line 199 "src/parse-gram.h"
 } YYSTYPE;
 # define YYSTYPE_IS_TRIVIAL 1