configure.ac need AC_PROG_SED macro

[snowflake]

My system is x86_64-apple-darwin15.6.0 (macOS 10.11.6)

As you can see. it is quite out of date.
I use autoconf 2.72 and automake 1.17
Both are installed with Macports.
I don't use Homebrew.

Git checkout is 535c64a
This is the master as of today.

Everything mostly builds OK, except for
the doc/Makefile. The SED variable is not defined so that

mactelnetd.1: mactelnetd.1.in
        $(AM_V_GEN)$(SED) -e 's|@sysconfdir[@]|$(sysconfdir)|g' mactelnetd.1.in > $@

does not work. mactelnetd.1 is a zero-length file.

I fixed the problem by placing AC_PROG_SED in configure.ac
I don't know the approved location for this macro as my knowledge of autoconf and automake
is not great.

I have another Mac running x86_64-apple-darwin24.2.0 ( macOS 15.2).
It has the same versions of autoconf and automake as my old system.
It works without AC_PROG_SED in configure.ac. I don't know why.