SDL_net: define _DARWIN_C_SOURCE only if not already defined

SDL Commits
1 
From 2931c14b70ac5ae5332d3c560be0d7bad51682f6 Mon Sep 17 00:00:00 2001
From: Ozkan Sezer <[EMAIL REDACTED]>
Date: Sat, 17 Apr 2021 21:56:50 +0300
Subject: [PATCH] define _DARWIN_C_SOURCE only if not already defined

---
 SDLnetsys.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/SDLnetsys.h b/SDLnetsys.h
index f8d0073..e7b79ca 100644
--- a/SDLnetsys.h
+++ b/SDLnetsys.h
@@ -20,7 +20,7 @@
 */
 
 /* Include normal system headers */
-#ifdef __APPLE__
+#if defined(__APPLE__) && !defined(_DARWIN_C_SOURCE)
 #define _DARWIN_C_SOURCE
 #endif
 #include <stdio.h>