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/fps_counter.h

@@ -1,14 +1,14 @@
 #ifndef FPSCOUNTER_H
 #define FPSCOUNTER_H
 
+#include "common.h"
+
 #include <stdatomic.h>
 #include <stdbool.h>
 #include <stdint.h>
 #include <SDL2/SDL_mutex.h>
 #include <SDL2/SDL_thread.h>
 
-#include "common.h"
-
 struct fps_counter {
     SDL_Thread *thread;
     SDL_mutex *mutex;