Pending upstream PR
https://github.com/indexdata/yaz/pull/163.patch
From b3214e59d168c1c9b348ca02bf443046a3ab834a Mon Sep 17 00:00:00 2001
From: Nicolas PARLANT <nicolas.parlant@parhuet.fr>
Date: Fri, 4 Jul 2025 15:26:55 +0200
Subject: [PATCH] fix musl - expose gethostbyaddr with _GNU_SOURCE

>tcpdchk.c:72:25: error: call to undeclared function 'gethostbyaddr'; ISO C99 and later do not support implicit function declarations
>      [-Wimplicit-function-declaration]
>   72 |             if ((host = gethostbyaddr((char*)&addr_in->sin_addr,
>      |                         ^

Signed-off-by: Nicolas PARLANT <nicolas.parlant@parhuet.fr>
---
 src/tcpdchk.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/src/tcpdchk.c b/src/tcpdchk.c
index f98417033..6e9a65adb 100644
--- a/src/tcpdchk.c
+++ b/src/tcpdchk.c
@@ -36,6 +36,8 @@
 #endif
 
 #if HAVE_NETDB_H
+/* _GNU_SOURCE: for musl's netdb.h to expose gethostbyaddr */
+#define _GNU_SOURCE
 #include <netdb.h>
 #endif