Bug#25042 OPTIMIZE TABLE cause race condition in IO CACHE SHARE

- The condition variable implementation "lost" a signal to
  WaitOnSingleObject when a semaphore was released.
- The signal could be consumed by a new call to pthread_cond_wait
  before all waiting threads had awoken.
- The new implementation of pthread_cond_* uses events
  instead of semaphores. It also uses an extra lock to protect entry
  into new cond wait before the broadcast has finished.


include/my_pthread.h:
  - New implementatin of pthread_cond_init. This version uses events
    instead of semaphores
mysys/my_wincond.c:
  - New implementatin of pthread_cond_init. This version uses events
    instead of semaphores

include/my_pthread.h

@@ -80,11 +80,17 @@ typedef struct st_pthread_link {
 
 typedef struct {
   uint32 waiting;
-#ifdef OS2
-  HEV    semaphore;
-#else
-  HANDLE semaphore;
-#endif
+  CRITICAL_SECTION lock_waiting;
+ 
+  enum {
+    SIGNAL= 0,
+    BROADCAST= 1,
+    MAX_EVENTS= 2
+  } EVENTS;
+
+  HANDLE events[MAX_EVENTS];
+  HANDLE broadcast_block_event;
+
 } pthread_cond_t;
 
 typedef int pthread_mutexattr_t;