It's not possible to use WaitForSingleObject to wait
on a CRITICAL_SECTION, instead use the TryEnterCriticalSection function.
 - if "mutex" was already taken => return EBUSY
 - if "mutex" was aquired => return 0


include/config-win.h:
  Make windows.h define TryEnterCriticalSection
mysys/my_winthread.c:
  Use the TryEnterCriticalSection function to implement pthread_mutex_trylock
  Prevent recursive behaviour by looking at the RecursionCount variable
  in the CRITICAL_SECTION struct and return EBUSY from function if
  the mutex was already locked once.