Initialize LOCK_open as a adapative mutex on platforms where the
PTHREAD_ADAPTIVE_MUTEX_INITIALIZER_NP macro is available. The flag
indicates that a thread should spin (busy wait) for some time on a
locked adaptive mutex before blocking (sleeping). It's intended to
to alleviate performance problems due to LOCK_open being a highly
contended mutex.

sql/mysqld.cc:
  Initialize LOCK_open as a adapative mutex.