* thread_pthread.c (ruby_init_stack): use stack info if possible.

	* thread_pthread.c (ruby_init_stack): adjust stack size for offset of
	  addr from the bottom.

	* thread_pthread.c (get_stack): seems stack size does not include
	  guard size on Mac OS X.

	* gc.h (IS_STACK_DIR_UPPER): utility macro.

	* thread_pthread.c (get_stack): Linux is the only OS which includes
	  the size of guard page into the stack size.

	* thread_pthread.c (ruby_init_stack): STACK_GROW_DIR_DETECTION is
	  necessary on platforms with unknown stack direction.  [Bug #6761]


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_1_9_3@36789 b2dd03c8-39d4-4d8f-98ff-823fe69b080e