info: using existing install for '1.20.0-x86_64-apple-darwin'
info: override toolchain for '/Users/benjyw/src/pants' set to '1.20.0-x86_64-apple-darwin'

  1.20.0-x86_64-apple-darwin unchanged - rustc 1.20.0 (f3d6973f4 2017-08-27)

generating /Users/benjyw/src/pants/src/rust/engine/src/cffi/native_engine.c
(already up-to-date)
generating /Users/benjyw/src/pants/src/rust/engine/src/cffi/native_engine.cflags
   Compiling protobuf v1.4.2
   Compiling grpcio-sys v0.1.1 (<https://github.com/illicitonion/grpc-rs?rev=2acabdbff71e77a14b95b0b30c6d7de516355df7#2acabdbf>)
   Compiling ignore v0.1.9
error: could not find native static library `z`, perhaps an -L flag is missing?

error: Could not compile `grpcio-sys`.
warning: build failed, waiting for other jobs to finish...
error: build failed

Failed to build native engine.