benchtests: Enable BIND_NOW if configured with --enable-bind-now

Benchmarks should reflect distribution build policies, so it makes
sense to honor the BIND_NOW configuration for them.

This commit keeps using $(+link-tests), so that the benchmarks are
linked according to the --enable-hardcoded-path-in-tests configure
option.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>