[libprotobuf WARNING google/protobuf/compiler/parser.cc:491] No syntax
specified for the proto file. Please use 'syntax = "proto2";' or
'syntax = "proto3";' to specify a syntax version. (Defaulted to proto2
syntax.)
The release notes say a future release will turn this warning into an
error.
Signed-off-by: Anders Kaseorg <andersk@mit.edu>