Fix some Wimplicit-fallthrough

A "fall through" comment is most portable way to indicate
"no break, fallthru" cases.

 * __attribute__ ((fallthrough))  // is not portable
 * [[fallthrough]]; // is C++17

libs/ardour/automatable.cc

@@ -333,7 +333,7 @@ Automatable::protect_automation ()
 			l->set_automation_state (Off);
 			break;
 		case Latch:
-			// no break
+			/* fall through */
 		case Touch:
 			l->set_automation_state (Play);
 			break;

libs/ardour/lua_api.cc

@@ -681,10 +681,11 @@ LuaTableRef::set (lua_State* L)
 					}
 					// invalid userdata -- fall through
 				}
-				// no break
+				/* fall through */
 			case LUA_TFUNCTION: // no support -- we could... string.format("%q", string.dump(value, true))
+				/* fall through */
 			case LUA_TTABLE: // no nested tables, sorry.
-			case LUA_TNIL: // fallthrough
+			case LUA_TNIL:
 			default:
 				// invalid value
 				lua_pop (L, 2);

libs/ardour/session_playlists.cc

@@ -400,8 +400,8 @@ SessionPlaylists::maybe_delete_unused (boost::function<int(boost::shared_ptr<Pla
 		case 2:
 			// delete this and all later
 			delete_remaining = true;
-			// no break;
 
+			/* fall through */
 		case 1:
 			// delete this
 			playlists_tbd.push_back (*x);

libs/evoral/src/Curve.cpp

@@ -438,8 +438,8 @@ Curve::multipoint_eval (double x) const
 					double x2 = x * x;
 					return ev->coeff[0] + (ev->coeff[1] * x) + (ev->coeff[2] * x2) + (ev->coeff[3] * x2 * x);
 				}
-				// no break, fallthru
-			default: // Linear
+				/* fall through */
+			case ControlList::Linear:
 				return before->value + (vdelta * (tdelta / trange));
 		}
 	}

libs/widgets/ardour_icon.cc

@@ -1037,8 +1037,10 @@ ArdourWidgets::ArdourIcon::render (cairo_t *cr,
 		case TransportPanic:
 			icon_transport_panic (cr, width, height);
 			break;
-		case TransportStart: // no break
-		case TransportEnd:   // no break
+		case TransportStart:
+			/* fall through */
+		case TransportEnd:
+			/* fall through */
 		case TransportRange:
 			icon_transport_ck (cr, icon, width, height);
 			break;
@@ -1063,8 +1065,10 @@ ArdourWidgets::ArdourIcon::render (cairo_t *cr,
 		case NudgeRight:
 			icon_nudge_right (cr, width, height, fg_color);
 			break;
-		case ZoomIn:  // no break
-		case ZoomOut: // no break
+		case ZoomIn:
+			/* fall through */
+		case ZoomOut:
+			/* fall through */
 		case ZoomFull:
 			icon_zoom (cr, icon, width, height, fg_color);
 			break;