Pass native output directories of dependencies

This is often useful for picking up things like headers files from `*-sys`
packages when they had to compile locally (or perhaps for pkg-config).

Closes #449
auto merge of #471 : alexcrichton/cargo/issue-449, r=brson

This is often useful for picking up things like headers files from `*-sys`
packages when they had to compile locally (or perhaps for pkg-config).

Closes #449