The method signature of Dependency changed in
1fdf69b90382c43493a5f62f0020729289db6c70, however LD64Dependency was
missed in the update. This resulted in failures in merge_repeats(),
which creates many dependency objects using all three arguments.
This allows formulae which won't build with Tiger's ld to conditionally
request a dependency on the ld64 formula. This modifies the build
environment appropriately, and will only be active on Tiger.