Compile error with GCC 13

[wangvsa]

Distributor ID: Ubuntu
Description: Ubuntu 24.04.3 LTS
Release: 24.04
Codename: noble

uname -a
Linux 281b889092aa 5.14.21-150500.55.65_13.0.74-cray_shasta_c_64k #1 SMP Mon Sep 9 09:48:48 UTC 2024 (ba86e71) aarch64 aarch64 aarch64 GNU/Linux

Issue 1:
testutil.c:22:12: error: static declaration of 'asprintf' follows non-static declaration

Adding #define _GNU_SOURCE seems solved the issue.

Issue 2:
The intentional double-fclose ‘bug’ now appears to be caught at compile time. We may want to remove it, or at least keep it only for older compilers.
t/std/fopen-fclose.c:102:10: error: pointer 'fd' used after 'fclose' [-Werror=use-after-free]

Similarly, in some other test code, we intentionally perform operations on closed file descriptors, but they are now flagged as compile-time errors.