dnsdist: Properly link with `libdl` when building with `autotools`

dnsdist: Properly link with `libdl` when building with `autotools`

Depending on the system we might actually need to link with `libdl`
when our Rust library is used, and the mechanism to do that was not
properly set up when building with `autotools` (we were adding `LIBDL`
to the the libraries we need but the variable was not properly filled).
Unfortunately the systems we are exercising in our CI do not need to
explicitly link with `libdl` so we did not notice.

Signed-off-by: Remi Gacogne <remi.gacogne@powerdns.com>