* sync.h (sync::init_lock): Declare new static member.

(sync::init()): Declare new static function.
* sync.cc (sync::init): Define.
(sync::init): Lock attempt to initialize a muto to stop multiple threads from
colliding.
* dcrt0.cc (dll_crt0_0): Initialize muto environment.

winsup/cygwin/dcrt0.cc

@@ -574,6 +574,7 @@ void __stdcall
 dll_crt0_0 ()
 {
   wincap.init ();
+  muto::init ();
   initial_env ();
 
   char zeros[sizeof (child_proc_info->zero)] = {0};