gthread: Count how many threads have been started

This will be used in a following commit to warn if setenv() is used
after another thread has been created.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

Helps: #715

glib/gthreadprivate.h

@@ -81,6 +81,8 @@ gboolean g_thread_get_scheduler_settings (GThreadSchedulerSettings *scheduler_se
 
 gpointer        g_thread_proxy                  (gpointer      thread);
 
+guint           g_thread_n_created              (void);
+
 gpointer        g_private_set_alloc0            (GPrivate       *key,
                                                  gsize           size);