Fix Debian package dependency when using GnuTLS < 3.6.0
Work around what is probably a bug in dpkg-shlibdeps; it sees the "libgnutls28-dev (>= 3.6.6) | libgnutls28-dev (<< 3.6.0)," in the build-dependencies not as two alternatives, but as an absolute dependency on libgnutls30 >= 3.6.6. So if we compiled with libgnutls30 << 3.6.0, we have to hide this build dependency.
* debian/rules (override_dh_shlibdeps-arch): New; conditionally edit debian/control before running dh_shlibdeps.