native_midi_linux_alsa.c: define _POSIX_C_SOURCE and _GNU_SOURCE

.. if not already defined

Fixes #668

Author: sezero

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>