Rust builds fail on macOS with linker error

Both my gcc and clang builds get this error: https://travis-ci.org/teor2345/tor/jobs/316748503 https://travis-ci.org/teor2345/tor/jobs/316748509

Undefined symbols for architecture x86_64:

  "_res_9_init", referenced from:

      std::sys::unix::net::res_init_if_glibc_before_2_26::h5cce5181e3d1daee in libtor_rust.a(std-af00cffd9617c4f1.std13-3c37598e0257c2e4b7713b0956fa5bab.rs.rcgu.o)

ld: symbol(s) not found for architecture x86_64

The Linux builds are fine, as are the non-Rust macOS builds.

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