[boost] Getting started guide updates

Folks, I've tried to update the getting started guide (for Unix so far) to better explain building, in particular: - To make the "bootstrap + bjam" process the only documented one, since it turns out to be fairly painless on Unix - To clarify some common questions inline The result can be previewed at http://lvk.cs.msu.su/~ghost/unix-variants.pdf , see section 5. I would appreciate if folks take a look and suggest whether it can be improved further. I plan to make a similar update for Windows, but it's somewhat tricker. Thanks, -- Vladimir Prus Mentor Graphics +7 (812) 677-68-40