cannot run make rust correctly if I didn't install rust-protobuf in advance

[Sheng Tang]

If I didn't install rust-protobuf and run "make rust" directly, I will get such infomation:

./generate_rust.sh
generate rust code...
protoc-gen-rust: program not found or is not executable
--rust_out: protoc-gen-rust: Plugin failed with status code 1.
make: *** [rust] Error 1

I think we could add some codes to generate_rust.sh.

if ! cmd_exists protoc-gen-rust; then
    echo "missing rust-protobuf, try to download/install it"
    cargo install protobuf
fi

[siddontang]

Thanks for your report @ts25504

Could you send us a PR? Maybe we should also consider installing gogo too :-)

[Sheng Tang]

I'll try to fix it.