on Windows".

On platforms where read-write lock implementation does not
prefer readers by default (Windows, Solaris) server might
have deadlocked while detecting MDL deadlock.

MDL deadlock detector relies on the fact that read-write
locks which are used in its implementation prefer readers
(see new comment for MDL_lock::m_rwlock for details).
So far MDL code assumed that default implementation of
read/write locks for the system has this property.
Indeed, this turned out ot be wrong, for example, for
Windows or Solaris. Thus MDL deadlock detector might have
deadlocked on these systems.

This fix simply adds portable implementation of read/write
lock which prefer readers and changes MDL code to use this
new type of synchronization primitive.

No test case is added as existing rqg_mdl_stability test can
serve as one.

config.h.cmake:
  Check for presence of pthread_rwlockattr_setkind_np to be
  able to determine if system natively supports read-write
  locks for which we can specify if readers or writers should
  be preferred.
configure.cmake:
  Check for presence of pthread_rwlockattr_setkind_np to be
  able to determine if system natively supports read-write
  locks for which we can specify if readers or writers should
  be preferred.
configure.in:
  Check for presence of pthread_rwlockattr_setkind_np to be
  able to determine if system natively supports read-write
  locks for which we can specify if readers or writers should
  be preferred.
include/my_pthread.h:
  Added support for portable read-write locks which prefer
  readers.
  To do so extended existing my_rw_lock_t implementation to
  support selection of whom to prefer depending on a flag.
mysys/thr_rwlock.c:
  Extended existing my_rw_lock_t implementation to support
  selection of whom to prefer depending on a flag.
  Added rw_pr_init() function implementing initialization of
  read-write locks preferring readers.
sql/mdl.cc:
  Use portable read-write locks which prefer readers instead of
  relying on that system implementation of read-write locks has
  this property (this was true for Linux/NPTL but was false,
  for example, for Windows and Solaris).
  Added comment explaining why preferring readers is important
  for MDL deadlock detector (thanks to Serg for example!).
sql/mdl.h:
  Use portable read-write locks which prefer readers instead of
  relying on that system implementation of read-write locks has
  this property (this was true for Linux/NPTL but was false,
  for example, for Windows and Solaris).