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/gsettingsbackend.c

@@ -26,7 +26,6 @@
 #include "gsettingsbackendinternal.h"
 #include "gsimplepermission.h"
 #include "giomodule-priv.h"
-#include "gio-marshal.h"
 
 #include <string.h>
 #include <stdlib.h>