Stop using glib-genmarshal at build time

To help cross compilation, don't use glib-genmarshal in our
build.  This is easy now that we have g_cclosure_marshal_generic().

In gobject/, add gmarshal.[ch] to git (making the existing entry
points stubs).

In gio/, simply switch to using g_cclosure_marshal_generic().

https://bugzilla.gnome.org/show_bug.cgi?id=652168

gio/gdbusinterface.c

@@ -24,7 +24,6 @@
 
 #include "gdbusobject.h"
 #include "gdbusinterface.h"
-#include "gio-marshal.h"
 
 #include "glibintl.h"