[boost] thread lib patch

Using MSVC6, STLport 5.0 beta, february 2003 SDK under Windows XP I failed to build thread library because of a missing include. In read_write_mutex.cpp, tchar.h contains _sntprintf macro definition but not the functions definition hidden behind this macro. Those are in stdio.h. Maybe this file was included in a previous version of STLport but it is not anymore. I am also surprised that those files are included if BOOST_HAS_WINTHREADS is defined and symbos coming from them are used if WINVER is defined. Isn't there a little inconsistency? Bests