AS_ECHO and printf

[Po Lu]

When did AS_ECHO start to emit calls to `printf', and `AS_UNSET' to
`unset'?  printf is not portable; it is missing, both as a built-in to
ksh and a command, on Ultrix 4.5, as well as (AFAIK) Solaris 2.6, the
first of which is important for historical purposes, and the second of
which is still found in the wild in some fields.

I can understand if Gnulib no longer considers these systems porting
targets.  But Autoconf is used by much more software than GNU software,
and should IMHO hold itself to a higher standard.

Thanks.