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/compat.h

@@ -1,6 +1,13 @@
 #ifndef COMPAT_H
 #define COMPAT_H
 
+#define _POSIX_C_SOURCE 200809L
+#define _XOPEN_SOURCE 700
+#define _GNU_SOURCE
+#ifdef __APPLE__
+# define _DARWIN_C_SOURCE
+#endif
+
 #include <libavcodec/version.h>
 #include <libavformat/version.h>
 #include <SDL2/SDL_version.h>