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.

data/skeletons/lalr1.cc

@@ -532,7 +532,10 @@ m4_if(b4_prefix, [yy], [],
             case '\\':
               if (*++yyp != '\\')
                 goto do_not_strip_quotes;
-              // Fall through.
+              else
+                goto append;
+
+            append:
             default:
               yyr += *yyp;
               break;