Since cargo-deb builds all members of the workspace by default, it is
necessary that the winpty subcrate can be built on all operating
systems, since it's not possible to have OS-specific workspace members.
To achieve this the crate has been changed to be empty by default on
non-windows systems. It might make sense to do something similar with
winpty-sys, but it's not strictly necessary at this point since we don't
directly depend on it.
This fixes#1716.
