Bug#55001 Change in behavior: thread_concurrency (docs,

          example files)

The system variable 'thread_concurrency' has been
(re-)enabled on all platforms, to prevent startup
errors.

'thread_concurrency' is unused and has no effect,
on any platform, in MySQL 5.1 and later versions.  It 
will be deprecated, and removed, in context of 
worklog WL#5265

mysql-test/include/have_thread_concurrency.inc

-disable_query_log;
-disable_result_log;
-set @have_thread_concurrency=0;
-select @have_thread_concurrency:=1 from information_schema.global_variables where variable_name='thread_concurrency';
-if (`select @have_thread_concurrency = 0`)
-{
-  skip Need @@thread_concurrency;
-}
-enable_result_log;
-enable_query_log;

mysql-test/suite/sys_vars/t/thread_concurrency_basic.test

---source include/have_thread_concurrency.inc
 #
 # only global
 #

sql/sys_vars.cc

@@ -1617,14 +1617,17 @@ static Sys_var_charptr Sys_socket(
        READ_ONLY GLOBAL_VAR(mysqld_unix_port), CMD_LINE(REQUIRED_ARG),
        IN_FS_CHARSET, DEFAULT(0));
 
-#ifdef HAVE_THR_SETCONCURRENCY
+/* 
+  thread_concurrency is a no-op on all platforms since
+  MySQL 5.1.  It will be removed in the context of
+  WL#5265
+*/
 static Sys_var_ulong Sys_thread_concurrency(
        "thread_concurrency",
        "Permits the application to give the threads system a hint for "
        "the desired number of threads that should be run at the same time",
        READ_ONLY GLOBAL_VAR(concurrency), CMD_LINE(REQUIRED_ARG),
        VALID_RANGE(1, 512), DEFAULT(DEFAULT_CONCURRENCY), BLOCK_SIZE(1));
-#endif
 
 static Sys_var_ulong Sys_thread_stack(
        "thread_stack", "The stack size for each thread",