Add glib__private__() API to share between glib,gio; port GWakeup to it

Historically we've added random symbols to the public API with warnings
that they're private; examples are:

glib_gettext(), glib_pgettext()
g_thread_functions_for_glib_use, g_thread_use_default_impl, etc.

And we almost added "GWakeup" to public API just to share between glib and
gio.

This new glib__private__() API exports a hidden vtable, and adds a macro
GLIB_PRIVATE_CALL() that makes it generally convenient to use.

This adds an extremely tiny cost for the double indirection; but it has
the benefit that we don't need to either:

1) compile the code into both glib and gio (like GWakeup), with the
   inefficiency that implies.
2) Export a "do not use this" symbol; the serious problem with this is
   that someone CAN use it pretty easily.  Particularly if we document
   it.  It's far, far harder to peek into a structure without a public
   header file.

https://bugzilla.gnome.org/show_bug.cgi?id=657992

glib/glib-private.h

@@ -0,0 +1,48 @@
+/* glib-private.h - GLib-internal private API, shared between glib, gobject, gio
+ * Copyright (C) 2011 Red Hat, Inc.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Library General Public
+ * License as published by the Free Software Foundation; either
+ * version 2 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Library General Public License for more details.
+ *
+ * You should have received a copy of the GNU Library General Public
+ * License along with this library; if not, write to the
+ * Free Software Foundation, Inc., 59 Temple Place - Suite 330,
+ * Boston, MA 02111-1307, USA.
+ */
+
+#ifndef __GLIB_PRIVATE_H__
+#define __GLIB_PRIVATE_H__
+
+#include <glib.h>
+
+#include "gwakeup.h"
+
+G_BEGIN_DECLS
+
+#define GLIB_PRIVATE_CALL(symbol) (glib__private__()->symbol)
+
+typedef struct {
+  /* See gwakeup.c */
+  GWakeup *   (*g_wakeup_new)         (void);
+  void        (*g_wakeup_free)        (GWakeup *wakeup);
+  void        (*g_wakeup_get_pollfd)  (GWakeup *wakeup,
+				       GPollFD *poll_fd);
+  void        (*g_wakeup_signal)      (GWakeup *wakeup);
+  void        (*g_wakeup_acknowledge) (GWakeup *wakeup);
+
+  /* Add other private functions here, initialize them in gutils.c */
+} GLibPrivateVTable;
+
+GLibPrivateVTable *glib__private__ (void);
+
+
+G_END_DECLS
+
+#endif /* __G_MAIN_H__ */