This change would break gnulib's gl_CACHE_VAL_SILENT. Is there some way you can fix that? It's OK if we have to modify gl_CACHE_VAL_SILENT, it's just that we need to modify it in a way that'll work with both old and new Autoconf.
Also, any such patch to Autoconf should update
the documentation a bit for "@item @command {echo}",
and it needs a NEWS item.
