fix automake-1.13 deprecations

and fix a build error.

DEBUG:   File "serializeimages.py", line 14, in <module>
DEBUG:     pixbuf64 = gtk.gdk.pixbuf_new_from_file("data/icons/hicolor/64x64/apps/dropbox.png")
DEBUG: glib.GError: Failed to open file 'data/icons/hicolor/64x64/apps/dropbox.png': No such file or directory
DEBUG: make[2]: *** [caja-dropbox] Error 1