c, c++: avoid implicit fall-throw

Reported by Derek Clegg.
http://lists.gnu.org/archive/html/bug-bison/2019-01/msg00004.html

* configure.ac (warn_common): Add -Wimplicit-fallthrough.
This does trigger failures in the test suite.
* data/skeletons/glr.c, data/skeletons/lalr1.cc,
* data/skeletons/yacc.c, tests/c++.at:
Make fall-throws explicit.

tests/c++.at

@@ -1242,7 +1242,8 @@ yylex (yy::parser::semantic_type *lvalp)
   default:
     lvalp->]AT_VARIANT_IF([build<Object> (res)],
                           [obj = new Object (res)])[;
-    // Fall through.
+    goto zero;
+  zero:
   case 0:
     return res;
   }