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Now thd->mem_root is a pointer to thd->main_mem_root and THR_MALLOC is a pointer to thd->mem_root.
This gives us the following benefits:
- Allow us to easily detect if arena has already been swapped before (this fixes a bug in setup_conds() where arena was swaped twice in some cases)
- Faster swaps of arenas (as we don't have to copy the whole MEM_ROOT)
- We don't anymore have to call my_pthread_setspecific_ptr(THR_MALLOC,...) to change where memory is alloced. Now it's enough to set thd->mem_root



client/mysqltest.c:
  Remove some not needed defines
  (Things like this should be done in config-win.h)
include/config-win.h:
  Added popen() and pclose() compatibility macros
mysql-test/t/rpl_failed_optimize-master.opt:
  Portability fix
sql/ha_berkeley.cc:
  New thd->memroot handling
sql/item_cmpfunc.cc:
  Simpler arena swapping code
sql/item_func.cc:
  Simpler arena swapping code
sql/item_subselect.cc:
  Simpler arena swapping code
  New thd->mem_root handling
sql/item_sum.cc:
 ...