https://github.com/openSUSE/xinetd/pull/42

From 0539bd4450020c83c6b72eec1004e3ce3f64554b Mon Sep 17 00:00:00 2001
From: Sam James <sam@gentoo.org>
Date: Sat, 10 Sep 2022 13:59:20 +0100
Subject: [PATCH 1/2] configure.ac: use AC_USE_SYSTEM_EXTENSIONS

We use fcvt and gcvt, both of which are extensions
guarded by GNU_SOURCE. Use AC_USE_SYSTEM_EXTENSIONS
to ensure they're always available.

Fixes a build failure on musl with Clang 15:
```
src/sio/sprint.c:176:15: error: call to undeclared function 'fcvt'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
                p = (char *)fcvt( num, precision, &decimal_point, is_negative ) ;
src/sio/sprint.c:178:15: error: call to undeclared function 'ecvt'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
                p = (char *)ecvt( num, precision+1, &decimal_point, is_negative ) ;
```

And drop a single #define _GNU_SOURCE from signals.c, as it's
preferable to have it consistently defined across the codebase
where possible. We have autotools, so let's use it.
--- a/configure.ac
+++ b/configure.ac
@@ -38,6 +38,8 @@ AC_CHECK_LIB([m], [log10], [
 ])
 AC_SUBST([LIBM_LIBS])
 
+AC_USE_SYSTEM_EXTENSIONS
+
 # ======================================
 # Check for various headers and settings
 # ======================================
--- a/src/signals.c
+++ b/src/signals.c
@@ -7,7 +7,6 @@
 
 
 #include "config.h"
-#define _GNU_SOURCE
 #include <sys/types.h>
 #include <sys/time.h>
 #include <signal.h>