link to librt and libdl only when needed

According to dpkg-shlibdeps, the three binaries in tor, tor-resolve, tor-gencert, and tor, link to libdl.so.2 and librt.so.1. without using any of their symbols.

dpkg-shlibdeps: warning: dependency on libdl.so.2 could be avoided if "debian/tor/usr/bin/tor-resolve debian/tor/usr/sbin/tor debian/tor/usr/bin/tor-gencert" were not uselessly linked against it (they use none of its symbols).
dpkg-shlibdeps: warning: dependency on librt.so.1 could be avoided if "debian/tor/usr/bin/tor-resolve debian/tor/usr/sbin/tor debian/tor/usr/bin/tor-gencert" were not uselessly linked against it (they use none of its symbols).

Why do we link against them? Should we?