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/main.c

@@ -1,5 +1,7 @@
 #include "scrcpy.h"
 
+#include "common.h"
+
 #include <assert.h>
 #include <stdbool.h>
 #include <unistd.h>
@@ -7,7 +9,6 @@
 #define SDL_MAIN_HANDLED // avoid link error on Linux Windows Subsystem
 #include <SDL2/SDL.h>
 
-#include "common.h"
 #include "cli.h"
 #include "util/log.h"