libtorrent, rtorrent: build using libstdc++

This fixes the libtorrent and rtorrent builds by making both of them use libstdc++, since libtorrent can't be built against libc++ at the moment.

Unfortunately this means rtorrent has to carry a private copy of libsigc++ - that lib is too widely used for us to force libstdc++ all the time, and rtorrent can't build without it. Hopefully this situation doesn't last too long.

Fixes #23483.