Build: do not error on fallthrough

.travis.yml

@@ -113,7 +113,7 @@ matrix:
               - p7zip-full
               - gcc-7
           compiler: gcc-7
-          env: WFLAGS="-Wno-tautological-compare"
+          env: WFLAGS="-Wno-error=implicit-fallthrough"
           #gcc-7 (Ubuntu 7.2.0-1ubuntu1~14.04) 7.2.0 20170802
         - os: linux
           compiler: clang

src/m_misc.c

@@ -1079,7 +1079,7 @@ void M_StartMovie(void)
 				moviemode = M_StartMovieGIF(pathname);
 				break;
 			}
-			// fall thru
+			/* FALLTHRU */
 		case MM_APNG:
 			moviemode = M_StartMovieAPNG(pathname);
 			break;