lalr1.cc: syntax_error as exceptions.

It is common to use sort of factories in the user actions.  These
factories may check some "syntactic" constraints that are not
enforced by the grammar itself.  This is possible using YYERROR
within the action itself.  Provide the user with a means to throw
a syntax_error exception.

	* data/c++.m4 (b4_public_types_declare, b4_public_types_define):
	Declare and define yy::parser::syntax_error.
	* data/lalr1.cc: Include stdexcept.
	(yy::parser::parse): Wrap the user action within a try/catch.
	* data/glr.cc: Include stdexcept.

data/lalr1.cc

@@ -147,6 +147,7 @@ dnl FIXME: This is wrong, we want computed header guards.
 ]b4_percent_code_get([[requires]])[
 
 ]b4_parse_assert_if([#include <cassert>])[
+#include <stdexcept>
 #include <string>
 #include <iostream>
 #include "stack.hh"
@@ -814,12 +815,21 @@ m4_ifdef([b4_lex_param], [, ]b4_lex_param)));])[
 
     // Perform the reduction.
     YY_REDUCE_PRINT (yyn);
-    switch (yyn)
+    try
+    {
+      switch (yyn)
       {
 ]b4_user_actions[
 	default:
           break;
       }
+    }
+    catch (const syntax_error& yyexc)
+    {
+      error (]b4_args(b4_locations_if([yyexc.location]),
+                      [[yyexc.what()]])[);
+      YYERROR;
+    }
     YY_SYMBOL_PRINT ("-> $$ =", yylhs);
 ]b4_variant_if([[
     // Destroy the rhs symbols.