mods to boost thread

* Fix bug where BOOST_PTHREAD_HAS_TIMEDLOCK is defined differently based on the include ordering, thus causing a violation of C++'s One Definition Rule (ODR). In particular, this define triggers the later conditional inclusion or exclusion of two members of class timed_mutex in mutex.hpp.
* NOTE: slightly different than how we [patched boost 1.49](https://github.com/smanders/boost-svn/commit/79b74847d79e7e358a56c0309e950597ce6ba92d) - there's now an `#ifndef BOOST_PTHREAD_HAS_TIMEDLOCK`