Windows improvements : manual backport of

htttp://lists.mysql.com/commits/50957?f=plain
      
Always use TLS functions instead of __declspec(thread) to access 
thread local storage variables.
The change removes the necessity to recomplile the same source
files twice -  with USE_TLS for DLLs and without USE_TLS for EXEs.
Real benefit of this change is better readability and maintainability
of TLS functions within MySQL.
              
There is a performance loss using TlsXXX functions compared to __declspec 
but the difference is negligible in practice. In a sysbench-like benchmark 
I ran with with TlsGetValue, pthread_[get|set]_specific was called 600000000 
times and took 0.17sec of total 35min CPU time, or 0.008%.

mysys/my_winthread.c

@@ -129,12 +129,5 @@ void pthread_exit(void *a)
   _endthread();
 }
 
-/* This is neaded to get the macro pthread_setspecific to work */
-
-int win_pthread_setspecific(void *a,void *b,uint length)
-{
-  memcpy(a,b,length);
-  return 0;
-}
 
 #endif