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/util/queue.h

@@ -2,12 +2,12 @@
 #ifndef QUEUE_H
 #define QUEUE_H
 
+#include "common.h"
+
 #include <assert.h>
 #include <stdbool.h>
 #include <stddef.h>
 
-#include "common.h"
-
 // To define a queue type of "struct foo":
 //    struct queue_foo QUEUE(struct foo);
 #define QUEUE(TYPE) { \