BTW, I was told that if boost/config.hpp is included before any system headers, it will still be able to use _REENTRANT to detect if -pthread was specified on the command line.
Yep, but if -pthread is specified on the command line, then presumably the user really does want thread safe code :-)
Right. The current problem is that _REENTRANT is always defined by libstdc++ headers, so we can't use it to detect if user has specified -pthread. However, if boost/config.hpp is included before system headers and checks for _REENTRANT before including system headers on its own, it can reliably check if -pthread was specified. Of course, the requirements to include boost/config.hpp before everything else is a drastic one.
Sorry, I misunderstood you: that was what the fix to libstdcpp3.hpp was for: it checks to see if libstdc++ is multithreaded and then sets Boost to the same default. This results in consistent behaviour irrespective of what headers you include and in what order you include them, but at the possible expense of turning on threading support when you don't really need it. It's not ideal, but it's going to have to suffice for 1.33. John.