use protobuf-src crate to vendor protoc C dependency

Author: dvdsk

candle-onnx/Cargo.toml

@@ -16,6 +16,7 @@ prost = "0.12.1"
 
 [build-dependencies]
 prost-build = "0.12.1"
+protobuf-src = "2.1.1"
 
 [dev-dependencies]
 anyhow = { version = "1", features = ["backtrace"] }

candle-onnx/build.rs

@@ -1,6 +1,7 @@
 use std::io::Result;
 
 fn main() -> Result<()> {
+    std::env::set_var("PROTOC", protobuf_src::protoc());
     prost_build::compile_protos(&["src/onnx.proto3"], &["src/"])?;
     Ok(())
 }