SDL_mixer: native_midi_linux_alsa.c: define _POSIX_C_SOURCE and _GNU_SOURCE

SDL Commits
1 
From 1b1d7bacf8b6b28e7d9403f9ce482a49c7db2183 Mon Sep 17 00:00:00 2001
From: Ozkan Sezer <[EMAIL REDACTED]>
Date: Fri, 28 Mar 2025 14:20:50 +0300
Subject: [PATCH] native_midi_linux_alsa.c: define _POSIX_C_SOURCE and
 _GNU_SOURCE

.. if not already defined

Fixes https://github.com/libsdl-org/SDL_mixer/issues/668
---
 src/codecs/native_midi/native_midi_linux_alsa.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/src/codecs/native_midi/native_midi_linux_alsa.c b/src/codecs/native_midi/native_midi_linux_alsa.c
index 21983817..da69e510 100644
--- a/src/codecs/native_midi/native_midi_linux_alsa.c
+++ b/src/codecs/native_midi/native_midi_linux_alsa.c
@@ -22,6 +22,14 @@
 #include <SDL3/SDL_platform.h>
 #ifdef SDL_PLATFORM_LINUX
 
+#ifndef _POSIX_C_SOURCE
+#define _POSIX_C_SOURCE 200809L
+#endif
+
+#ifndef _GNU_SOURCE
+#define _GNU_SOURCE 1
+#endif
+
 #include <SDL3/SDL_hints.h>
 #include <SDL3/SDL_atomic.h>