[cctbxbb] --enable-boost-threads flag

[Luc Bourhis]

Actually, Richard pointed out to me privately that as part of removing the support of Boost.Thread, I also remove the option -DBOOST_DISABLE_THREADS. Without it, some part of Boost supports threading, whatever that means. I did that on purpose, just to see if something would break, although I forgot to mention that in the commit message. It would again be very useful to know the full stacktrace to see whether that could be the problem.

Luc