Migrate to autoconf 2.72?

Hi Hackers,

I wanted to add “—enable-shadows-warning” to “./configure”, I updated configure.ac, then installed autoconf on my MacBook, then when I tried to run autoconf, I got an error:

```
% autoconf
configure.ac:22: error: Autoconf version 2.69 is required.
Untested combinations of 'autoconf' and PostgreSQL versions are not
recommended.  You can remove the check from 'configure.ac' but it is then
your responsibility whether the result works or not.
configure.ac:22: the top level
autom4te: error: /opt/homebrew/opt/m4/bin/m4 failed with exit status: 1
```

I saw my newly installed autoconf is version 2.72. Then I tried to install autoconf 2.69, it says 2.69 is dead now:
```
==> Fetching downloads for: autoconf@2.69
Error: autoconf@2.69 has been disabled because it is a versioned formula! It was disabled on 2024-12-14.
```

Then I tried to disable the version check, and got the other error:
```
% autoconf
configure.ac:1290: warning: $as_echo is obsolete; use AS_ECHO(["message"]) instead
lib/m4sugar/m4sh.m4:690: _AS_IF_ELSE is expanded from...
lib/m4sugar/m4sh.m4:697: AS_IF is expanded from...
./lib/autoconf/general.m4:2249: AC_CACHE_VAL is expanded from...
./lib/autoconf/general.m4:2270: AC_CACHE_CHECK is expanded from...
config/ax_pthread.m4:88: AX_PTHREAD is expanded from...
lib/m4sugar/m4sh.m4:697: AS_IF is expanded from...
configure.ac:1290: the top level
```

Looks like some efforts have to be made to upgrade to autoconf 2.72 (or another version), which is not a task I have confident to take. So I’m just reporting the problem, hope someone can take care of it.

Best regards,
--
Chao Li (Evan)
HighGo Software Co., Ltd.
https://www.highgo.com/