[boost] bbv2 bison/yacc windows unistd.h - Build Problems

I am trying to build the boost library from cvs HEAD, using bbv2. The problem platform is windows. (On linux everything works ok.) I've noticed that bbv2 uses more tools than bbv1 did. I.e. the bison/yacc within the graph library. So I installed bison, but during build, when the compiler tries to translate the file built by bison, it does not find unistd.h. This file is not available in the standard include path of the compiler used (msvc-7.1) but in a subdirectory of the bison path. How can I now tell bbv2 to use this directory? Sorry if my question sounds silly, but I am still rather new to bbv2. I guess I'd need to tweak site-config.jam? If yes what shall I put in there? Thank you for help. Roland