gcc -pthread should define _REENTRANT?

["Steve M. Robbins" <steven.robbins@videotron.ca>]

Hi,

I need some advice: how should one detect that the "-pthread" 
flag has been used?  Boost looks for one of the two preprocessor
symbols to be defined:

	_REENTRANT
	_PTHREADS

That strategy seems to work for 6 of 11 architectures.  However, boost
fails to build on: alpha, s390, ia64, hppa, and arm.  On the
architectures for which I could find a sid chroot, I verified that
_REENTRANT is not defined (gcc -x c /dev/null -E -dM -pthread | grep REENT)

Do these architectures support pthreads or not?  If so, do they define
a magic preprocessor symbol?

Thanks,
-Steve