Move constructor macros to an internal header and into generated code

With this we're not longer exporting the constructor headers, which means
we're not tying ourselves to a macro that might need special tweaking on
a compiler-by-compiler basis.

gio/tests/resources.c

@@ -19,6 +19,7 @@
  */
 
 #include <gio/gio.h>
+#include "gconstructor.h"
 #include "test_resources2.h"
 
 static void