boost: thread unsafe and thread safe by default

tldr some require boost built with thread unsafe. There's no way to do that with homebrew.

Nearly a year ago, ticket [8928](https://github.com/mxcl/homebrew/pull/8928) was opened to address this very issue. The solution proposed was to ad --with-thread-unsafe option.

Rather than adding an option, it appears the formula was changed to allow both thread unsafe and thread save versions of boost to be built by default.

Seems this change fell through the cracks over the past year, I re-added it.