curl-compilers: enable -Wimplicit-fallthrough=4 for GCC

This enables level 4 instead of the default level 3, which of the
currently used comments only allows /* FALLTHROUGH */ to silence the
warning.

Closes https://github.com/curl/curl/pull/2747

src/tool_getparam.c

@@ -1834,7 +1834,7 @@ ParameterError getparameter(const char *flag, /* f or -long-flag */
         config->default_node_flags = toggle?GETOUT_USEREMOTE:0;
         break;
       }
-      /* fall-through! */
+      /* FALLTHROUGH */
     case 'o': /* --output */
       /* output file */
     {

src/tool_urlglob.c

@@ -114,7 +114,7 @@ static CURLcode glob_set(URLGlob *glob, char **patternp,
       if(multiply(amount, pat->content.Set.size + 1))
         return GLOBERROR("range overflow", 0, CURLE_URL_MALFORMAT);
 
-      /* fall-through */
+      /* FALLTHROUGH */
     case ',':
 
       *buf = '\0';
@@ -157,7 +157,7 @@ static CURLcode glob_set(URLGlob *glob, char **patternp,
         ++pattern;
         ++(*posp);
       }
-      /* intentional fallthrough */
+      /* FALLTHROUGH */
     default:
       *buf++ = *pattern++;              /* copy character to set element */
       ++(*posp);