fix build and suppress offsetof warnings in backwards-compatibility code

git-svn-id: http://svn.code.sf.net/p/libtorrent/code/trunk@10149 a83610d8-ad2a-0410-a6ab-fc0612d85776
merged fix from RC_1_0

git-svn-id: http://svn.code.sf.net/p/libtorrent/code/trunk@10151 a83610d8-ad2a-0410-a6ab-fc0612d85776