Define feature test macros in common.h

This enables necessary functions once for all.

As a consequence, define common.h before any other header.

app/src/stream.h

@@ -1,13 +1,14 @@
 #ifndef STREAM_H
 #define STREAM_H
 
+#include "common.h"
+
 #include <stdbool.h>
 #include <stdint.h>
 #include <libavformat/avformat.h>
 #include <SDL2/SDL_atomic.h>
 #include <SDL2/SDL_thread.h>
 
-#include "common.h"
 #include "util/net.h"
 
 struct video_buffer;