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.

configure.ac

@@ -82,7 +82,7 @@ AC_ARG_ENABLE([gcc-warnings],
 AM_CONDITIONAL([ENABLE_GCC_WARNINGS], [test "$enable_gcc_warnings" = yes])
 if test "$enable_gcc_warnings" = yes; then
   warn_common='-Wall -Wextra -Wno-sign-compare -Wcast-align -Wdocumentation
-    -Wformat -Wpointer-arith -Wwrite-strings'
+    -Wformat -Wnull-dereference -Wpointer-arith -Wwrite-strings'
   warn_c='-Wbad-function-cast -Wshadow -Wstrict-prototypes'
   warn_cxx='-Wnoexcept'
   # Warnings for the test suite only.