[php-src] Issue #20845: Building `--with-fpm` without `--prefix` adds `NONE` to include path in `php-fpm.conf.default`

Issue: https://github.com/php/php-src/issues/20845
Author: Ninos

### Description

There's a small bug while building php-fpm, but with no prefix definition. After build process you get an  `/usr/local/etc/php-fpm.conf.default`, but the include path is wrong;
```ini
# current
include=NONE/etc/php-fpm.d/*.conf

# should
include=/usr/local/etc/php-fpm.d/*.conf
```

To solve this, you can simply run `./configure` with  `--prefix=/usr/local` flag.

I searched a bit in the code & think there's a missing default setting in C, but I'm too stupid for that language :-) Found out following lines of code, may this will help a bit for faster debugging :-)
https://github.com/php/php-src/blob/master/sapi/fpm/php-fpm.conf.in#L142
https://github.com/php/php-src/blob/master/sapi/fpm/config.m4#L502
https://github.com/php/php-src/blob/master/main/build-defs.h.in#L28

PS: There's an old workaround in current official docker images for that, seems they were too frustrated a) trying a rebuild with `prefix` flag  & b) opening an issue :D
https://github.com/docker-library/php/blob/master/8.5/alpine3.23/fpm/Dockerfile#L211
PS²: Is it possible to generate the config-files without ´.default` suffix? :)

### PHP Version

```plain
even before php8 era
```

### Operating System

_No response_