(pthread_mutex_init): Declare as a "weak" symbol.

(Logical change 1.71)
diff --git a/include/internal.h b/include/internal.h
index 9e6ca25..8bfbf65 100644
--- a/include/internal.h
+++ b/include/internal.h
@@ -83,6 +83,7 @@
    unconditionally and if -lpthread is around, they'll call the
    corresponding routines otherwise, they do nothing.  */
 
+#pragma weak pthread_mutex_init
 #pragma weak pthread_mutex_lock
 #pragma weak pthread_mutex_unlock