Attached is the output of "gcc -ansi -dM". I hope it contains enough information to answer your questions. Note also that _REENTRANT is not defined here, therefore its source must be somewhere outside of gcc.
I've been trying to reproduce this using HP's test drive machines, unfortunately they all use single-threaded gcc :-(
From all that's been said it's apparent that _REENTRANT is getting set by one of the gcc headers, but which one? This is important because we have a consistency problem otherwise (threading support getting turned on depending on which headers you happen to have included).
I've attached an updated version of boost/config/stdlib/libstdcpp3.hpp, can you see if this fixes the issue? If it does then either commit it or let me know and I'll commit it. John.