build: add -Wimplicit-fallthrough

This warning seems to be designed well. It doesn't seem to warn on
fallthrough-only case statements, so it's compatible to well written
code.

stream_dvdnav.c had an obscure bug in inactive code, fix it.
stream_dvb.c is the only place where it intentionally falls through, I
guess I'll just leave it alone.

stream/stream_dvdnav.c

@@ -475,6 +475,7 @@ static int control(stream_t *stream, int cmd, void *arg)
             break;
         if (dvdnav_angle_change(dvdnav, new_angle) != DVDNAV_STATUS_OK)
             return 1;
+        break;
     }
     case STREAM_CTRL_GET_LANG: {
         struct stream_lang_req *req = arg;

waftools/detections/compiler.py

@@ -41,7 +41,9 @@ def __add_generic_flags__(ctx):
                                  "-Werror=format-security",
                                  "-Wno-redundant-decls",
                                  "-Wvla",
-                                 "-Wno-format-truncation"])
+                                 "-Wno-format-truncation",
+                                 "-Wimplicit-fallthrough",
+                                 ])
 
 def __add_gcc_flags__(ctx):
     ctx.env.CFLAGS += ["-Wall", "-Wundef", "-Wmissing-prototypes", "-Wshadow",