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Usage of pthread_attr_setstacksize should be conditional on
the definition of _POSIX_THREAD_ATTR_STACKSIZE, according to
POSIX. Errors from pthread_attr_setstacksize should be
reported (POSIX lists EINVAL as a possible error).

I think PTHREAD_STACK_MIN should be considered. 

The documentation should list availibility of the feature,
currently Win32, OS/2, and POSIX threads (with the TSS
option, to be precise). If some platforms have specific
additional requirements on the possible values (eg. must be
a multiple of the page size), these should be documented, as
well.

Apart from that, the patch looks fine.