Avoid compiler warnings

At least GCC 7.3, with -O1 or -O2 (but not -O0 or -O3) generates
warnings with -Wnull-dereference when using yyformat: it fails to see
yyformat cannot be null.

Reported by Frank Heckenbach, https://savannah.gnu.org/patch/?9620.

* configure.ac: Use -Wnull-dereference if supported.
* data/glr.c, data/lalr1.cc, data/yacc.c: Define yyformat in such
a way that GCC cannot not see that yyformat is defined.
Using `default: abort();` also addresses the issue, but forces
the inclusion of `stdlib.h`, which we avoid.

data/lalr1.cc

@@ -1077,6 +1077,7 @@ b4_error_verbose_if([state_type yystate, const symbol_type& yyla],
         case N:                               \
           yyformat = S;                       \
         break
+      default: // Avoid compiler warnings.
         YYCASE_ (0, YY_("syntax error"));
         YYCASE_ (1, YY_("syntax error, unexpected %s"));
         YYCASE_ (2, YY_("syntax error, unexpected %s, expecting %s"));