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/cbuf.h

@@ -2,11 +2,11 @@
 #ifndef CBUF_H
 #define CBUF_H
 
+#include "common.h"
+
 #include <stdbool.h>
 #include <unistd.h>
 
-#include "common.h"
-
 // To define a circular buffer type of 20 ints:
 //     struct cbuf_int CBUF(int, 20);
 //