libssl 0.9.8k-6 disabled autorenegotation, and the -dev package
introduced the SSL3_FLAGS_ALLOW_UNSAFE_LEGACY_RENEGOTIATION flag.

Since we now set that flag if available we want to make sure that it
*is* available when building.

Therefore build-depend on libssl-dev >= 0.9.8k-6.

If we build against earlier versions we will not work once libssl gets
upgraded to a version that disabled renegotiations.