(spin-loop inside pthread_mutex_lock).

thr_mutex.c:
  Added spinloop in mutex_lock
my_pthread.h:
  Added definition of my_pthread_fastmutex_t
configure.in:
  Added --with-fast-mutexes switch


configure.in:
  Added --with-fast-mutexes switch
include/my_pthread.h:
  Added definition of my_pthread_fastmutex_t
mysys/thr_mutex.c:
  Added spinloop in mutex_lock