warning: avoid warnings about unreachable code

Reported by Uxio Prego.
https://lists.gnu.org/archive/html/help-bison/2018-11/msg00031.html

We also need to move the unreachable 'goto' to a reachable place,
otherwise clang complains about the code being unreachable anyway.
See also https://bugs.llvm.org/show_bug.cgi?id=39736.

Interestingly, we don't have to apply that trick to
`#define YYCDEBUG if (false) std::cerr`, clang does not warn when the
code comes from macro expansion.

* configure.ac: Use -Wunreachable-code when supported.
* data/lalr1.cc, data/yacc.c: Pacify clang's warning about `if (0)`
by using a macro.
Another possibility was to move this statement to a reachable place.
* tests/actions.at, tests/c++.at: Avoid generating unreachable code.

tests/c++.at

@@ -435,8 +435,6 @@ namespace yy
            return parser::token::TEXT;]])[
          }
     }
-
-    abort ();
   }
 }