build: re-enable compiler warnings, and fix them

There are warnings (-Wextra) in generated C++ code:

  ltlparse.cc: In member function 'ltlyy::parser::symbol_number_type
  ltlyy::parser::by_state::type_get() const':
  ltlparse.cc:452:33: warning: enumeral and non-enumeral type in
  conditional expression
      return state == empty_state ? empty_symbol : yystos_[state];

Reported by Alexandre Duret-Lutz.

It turns out that -Wall and -Wextra were disabled because of a stupid
typo.

* configure.ac: Fix the stupid typo.
* data/lalr1.cc, src/AnnotationList.c, src/InadequacyList.c,
* src/ielr.c, src/print.c, src/scan-code.l, src/symlist.c,
* src/symlist.h, src/symtab.c, src/tables.c, tests/actions.at,
* tests/calc.at, tests/cxx-type.at, tests/glr-regression.at,
* tests/named-refs.at, tests/torture.at:
Fix warnings, mostly issues about variables used only with assertions,
which are disabled with -DNDEBUG.

data/lalr1.cc

@@ -149,10 +149,11 @@ b4_variant_if([m4_include(b4_pkgdatadir/[variant.hh])])
 m4_define([b4_shared_declarations],
 [b4_percent_code_get([[requires]])[
 ]b4_parse_assert_if([# include <cassert>])[
-# include <vector>
+# include <cstdlib> // std::abort
 # include <iostream>
 # include <stdexcept>
-# include <string>]b4_defines_if([[
+# include <string>
+# include <vector>]b4_defines_if([[
 # include "stack.hh"
 ]b4_bison_locations_if([[# include "location.hh"]])])[
 ]b4_variant_if([b4_variant_includes])[
@@ -569,7 +570,10 @@ m4_if(b4_prefix, [yy], [],
   ]b4_parser_class_name[::symbol_number_type
   ]b4_parser_class_name[::by_state::type_get () const
   {
-    return state == empty_state ? empty_symbol : yystos_[state];
+    if (state == empty_state)
+      return empty_symbol;
+    else
+      return yystos_[state];
   }
 
   inline
@@ -622,6 +626,10 @@ m4_if(b4_prefix, [yy], [],
     std::ostream& yyoutput = yyo;
     YYUSE (yyoutput);
     symbol_number_type yytype = yysym.type_get ();
+    // Avoid a (spurious) G++ 4.8 warning about "array subscript is
+    // below array bounds".
+    if (yysym.empty ())
+      std::abort ();
     yyo << (yytype < yyntokens_ ? "token" : "nterm")
         << ' ' << yytname_[yytype] << " ("]b4_locations_if([
         << yysym.location << ": "])[;