[php-src] Issue #21816: autoconf 2.73 defaults to C23; breaks php <= 8.2

Issue: https://github.com/php/php-src/issues/21816
Author: BjarneDMat

### Description

The autoconf port was just updated to 2.73. One of the changes in this version is that it defaults to C23 if the compiler supports it. Xcode 26 clang supports C23 as do newer MacPorts clangs.

C23 is not fully backward compatible with older C standards. When using `autoreconf` - as we do in MacPorts for php >= 8.0 - the build fails in `/ext/bcmath` w/ a lot of 
```
[-Wdeprecated-non-prototype]
warning: a function definition without a prototype is deprecated in all versions of C and is not supported in C23
```
I've got patches for this issue for php{74,8[012]} in [PR32175](https://github.com/macports/macports-ports/pull/32175)

### PHP Version

```plain
PHP 8.2.30 (cli) (built: Apr 19 2026 17:37:53) (NTS)
Copyright (c) The PHP Group
Zend Engine v4.2.30, Copyright (c) Zend Technologies
    with Zend OPcache v8.2.30, Copyright (c), by Zend Technologies
```

### Operating System

macOS 15.7.4 x86_64; Xcode 26.3