destroy a condition variable
pthread_cond_destroy() function frees the resources
allocated by the condition variable cond.
If successful, the
pthread_cond_destroy() function will
return zero, otherwise an error number will be returned to indicate the error.
pthread_cond_destroy() will fail if:
- The value specified by cond is invalid.
- The variable cond is locked by another thread.
pthread_cond_destroy() conforms to
ISO/IEC 9945-1:1996 (“POSIX.1”).