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/gpollableinputstream.c

@@ -24,7 +24,6 @@
 
 #include "gpollableinputstream.h"
 #include "gasynchelper.h"
-#include "gio-marshal.h"
 #include "glibintl.h"
 
 /**
@@ -272,7 +271,7 @@ static GSourceFuncs pollable_source_funcs =
   pollable_source_dispatch,
   pollable_source_finalize,
   (GSourceFunc)pollable_source_closure_callback,
-  (GSourceDummyMarshal)_gio_marshal_BOOLEAN__VOID,
+  (GSourceDummyMarshal)g_cclosure_marshal_generic,
 };
 
 /**