additions to boost units

apply operators.diff patch
* see [Ticket #7573](https://svn.boost.org/trac/boost/ticket/7573)

boost 1.49
* [commit](https://github.com/smanders/boost-svn/commit/17dd5d53ee4696d62f513b7aee5ecdb61cc1dd49) where we applied this to boost 1.49
* all boost 1.49 [patch commits](https://github.com/smanders/boost-svn/commits/sdl_1_49_0)