according to [1] we don't need to replace -O2 with -Os anymore in 0.11.x series of libtorrent. maybe you want to update the ebuild. [1] http://libtorrent.rakshasa.no/wiki/LibTorrentKnownIssues
libtorrent-0.11.8 does not have this replace anymore (we have the opposite)
oh.. sorry, didn't read the ebuild correct then.