If several threads call simultaneously pthread_mutex_lock() on a mutex
initialized with PTHREAD_MUTEX_INITIALIZER, the current automatic
intialization may result in several initializations of the mutex.
Serialize the automatic initialization with a global mutex to avoid
that.

This can be done only for automatic initialization, as
pthread_mutex_init() may be called from a non Xenomai thread, which
could not sleep on a cobalt mutex, and serializing with a plain Linux
mutex would cause a switch to secondary mode for pthread_mutex_lock() in
case of automatic initialization.

Using a condition variable would create a cancellation point, whereas
pthread_mutex_lock() and pthread_mutex_unlock() can not be cancellation
points.