Remove more compilation warnings in MinGW64 build

* src/w32.c (faccessat, map_w32_filename):
* src/w32fns.c (w32_wnd_proc):
* src/w32term.c (w32_horizontal_scroll_bar_handle_click)
(w32_scroll_bar_handle_click): Use FALLTHROUGH to avoid compiler
warnings with GCC 7 and later.

src/w32.c

@@ -3379,6 +3379,7 @@ map_w32_filename (const char * name, const char ** pPath)
 	      if ( ! left )
 		str[-1] = c;		/* replace last character of part */
 	      /* FALLTHRU */
+	      FALLTHROUGH;
 	    default:
 	      if ( left && 'A' <= c && c <= 'Z' )
 	        {
@@ -3958,6 +3959,7 @@ faccessat (int dirfd, const char * path, int mode, int flags)
 	      goto check_attrs;
 	    }
 	  /* FALLTHROUGH */
+	  FALLTHROUGH;
 	case ERROR_FILE_NOT_FOUND:
 	case ERROR_BAD_NETPATH:
 	  errno = ENOENT;

src/w32fns.c

@@ -4414,6 +4414,7 @@ w32_wnd_proc (HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)
 	  goto dflt;
 	}
       /* Fall through */
+      FALLTHROUGH;
 
     case WM_SYSCHAR:
     case WM_CHAR:
@@ -4676,6 +4677,7 @@ w32_wnd_proc (HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)
       if (w32_pass_extra_mouse_buttons_to_system)
 	goto dflt;
       /* else fall through and process them.  */
+      FALLTHROUGH;
     case WM_MBUTTONDOWN:
     case WM_MBUTTONUP:
     handle_plain_button:
@@ -4781,6 +4783,7 @@ w32_wnd_proc (HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)
 	  track_mouse_event_fn (&tme);
 	  track_mouse_window = hwnd;
 	}
+      FALLTHROUGH;
     case WM_HSCROLL:
     case WM_VSCROLL:
       if (w32_mouse_move_interval <= 0
@@ -4822,6 +4825,7 @@ w32_wnd_proc (HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)
       if (w32_pass_multimedia_buttons_to_system)
 	goto dflt;
       /* Otherwise, pass to lisp, the same way we do with mousehwheel.  */
+      FALLTHROUGH;
 
       /* FIXME!!!  This is never reached so what's the purpose?  If the
 	 non-zero return remark below is right we're doing it wrong all
@@ -5084,6 +5088,7 @@ w32_wnd_proc (HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)
     case WM_MOUSELEAVE:
       /* No longer tracking mouse.  */
       track_mouse_window = NULL;
+      FALLTHROUGH;
 
     case WM_ACTIVATEAPP:
     case WM_ACTIVATE:
@@ -5124,6 +5129,7 @@ w32_wnd_proc (HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)
 	      menu_free_timer = 0;
 	    }
 	}
+      FALLTHROUGH;
     case WM_MOVE:
     case WM_SIZE:
     command:
@@ -5162,6 +5168,7 @@ w32_wnd_proc (HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)
          fails (see bug#25875).  But if it fails, we want to find out
          about it, so let's leave 1000 for now.  */
       sleep (1000);
+      FALLTHROUGH;
 
     case WM_WINDOWPOSCHANGING:
       /* Don't restrict the sizing of any kind of frames.  If the window

src/w32term.c

@@ -4346,6 +4346,7 @@ w32_scroll_bar_handle_click (struct scroll_bar *bar, W32Msg *msg,
 	    SetScrollInfo (SCROLL_BAR_W32_WINDOW (bar), SB_CTL, &si, TRUE);
 	  }
 	/* fall through */
+	FALLTHROUGH;
       default:
 	emacs_event->kind = NO_EVENT;
 	return FALSE;
@@ -4460,6 +4461,7 @@ w32_horizontal_scroll_bar_handle_click (struct scroll_bar *bar, W32Msg *msg,
 	    SetScrollInfo (SCROLL_BAR_W32_WINDOW (bar), SB_CTL, &si, TRUE);
 	  }
 	/* fall through */
+	FALLTHROUGH;
       default:
 	emacs_event->kind = NO_EVENT;
 	return FALSE;