Result: jam still gives "Thread support is unavailable" errors. Attempt: added a line to boost/config/user.hpp: #define BOOST_HAS_THREADS Result: builds!
Just defining BOOST_HAS_THREADS probably won't get things working - you need to set the right compiler switches to ensure that the compiler really is building multithreaded code, however Jam should do this for you, so my guess is that the vacc toolset description is wrong. Maybe someone familiar with the compiler/platform can jump in here and indicate what the problem with vacc-tools.jam is.
Afterwards you have to manually set BOOST_HAS_THREADS as you discovered by yourself. This is because the configure script discovers that POSIX threads are available by the configure system doesn't set BOOST_HAS_THREADS itself. Maybe it should? You should ask John Maddock about it.
It's not supposed to set it - the config system will still check for _REENTRANT and/or _MT etc to see if the compiler is in multi-thread mode, if vacpp sets some other define then let me know and I'll update the config system. John Maddock http://ourworld.compuserve.com/homepages/john_maddock/index.htm