fpurge implicit declaration error glibc 2.41

[Emanuele Torre]

The 81d85842687 commit from 2025-11-13 introduced an fpurge (stdout) in
sig.c

When compiling using:
    $ gcc --version|sed q
    gcc (Gentoo 14.3.1_p20250801 p4) 14.3.1 20250801
    $ /usr/lib/ld-linux.so.2 --version|sed q
    ld.so (Gentoo 2.41-r6 (patchset 8)) stable release version 2.41.

I get the following error:
    ../sig.c: In function ‘throw_to_top_level’:
    ../sig.c:467:5: error: implicit declaration of function ‘fpurge’ 
[-Wimplicit-function-declaration]
      467 |     fpurge (stdout);
          |     ^~~~~~

Reading the fpurge(3) man page I see
    SYNOPSIS
           /* unsupported */
           #include <stdio.h>

           int fpurge(FILE *stream);

           /* supported */
           #include <stdio.h>
           #include <stdio_ext.h>

           void  __fpurge(FILE *stream);

Suggesting that glibc does not suport fpurge(), only __fpurge that is
defined in the stdio_ext.h header.

o/
 emanuele6