Add g_get_num_processors()

Based on a patch from John Cupitt <jcupitt@gmail.com>

Useful for thread pools which should scale to number of processors.

See https://bugzilla.gnome.org/show_bug.cgi?id=687223

https://bugzilla.gnome.org/show_bug.cgi?id=614930

glib/gthread.h

@@ -249,6 +249,9 @@ void            g_once_init_leave               (volatile void  *location,
   (g_once_init_leave((location), (gsize) (result)))
 #endif
 
+GLIB_AVAILABLE_IN_2_36
+guint          g_get_num_processors (void);
+
 G_END_DECLS
 
 #endif /* __G_THREAD_H__ */