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/opengl.h

@@ -1,11 +1,11 @@
 #ifndef SC_OPENGL_H
 #define SC_OPENGL_H
 
+#include "common.h"
+
 #include <stdbool.h>
 #include <SDL2/SDL_opengl.h>
 
-#include "common.h"
-
 struct sc_opengl {
     const char *version;
     bool is_opengles;