support for older boost versions

git-svn-id: http://libtorrent.googlecode.com/svn/trunk@9976 f43f7eb3-cfe1-5f9d-1b5f-e45aa6702bda
attempt to fix linux build

git-svn-id: http://libtorrent.googlecode.com/svn/trunk@9977 f43f7eb3-cfe1-5f9d-1b5f-e45aa6702bda
fix to the fix

git-svn-id: http://libtorrent.googlecode.com/svn/trunk@9978 f43f7eb3-cfe1-5f9d-1b5f-e45aa6702bda