Merged bug fixes from bleeding-edge.

git-svn-id: https://boost-log.svn.sourceforge.net/svnroot/boost-log/trunk/boost-log@818 ab6a8de6-4e00-4c3d-a860-8899c17d353f
Removed dependency on Boost.Thread in single-threaded builds.

git-svn-id: https://boost-log.svn.sourceforge.net/svnroot/boost-log/trunk/boost-log@820 ab6a8de6-4e00-4c3d-a860-8899c17d353f
Disabled defaulted functions on GCC prior to 4.6 due to bugs.

git-svn-id: https://boost-log.svn.sourceforge.net/svnroot/boost-log/trunk/boost-log@822 ab6a8de6-4e00-4c3d-a860-8899c17d353f