To close part of these loops, I confirmed that applying protocolbuffers/protobuf#1333 did fix this issue for me. The very old autogen tooling that protobuf had was creating the no longer relevant flag.

If folks want to use it in homebrew themselves, while we wait for the 3.6 release that is estimated to land later this month, you can add

  patch do
    url "https://patch-diff.githubusercontent.com/raw/google/protobuf/pull/1333.diff"
    sha256 "80bd40e7aa561cf22a34ba3aa86d7cd55bc866e689d6e4a633049f17ecf3fbdc"
  end

to your protobuf.rb formula. (If you don't know where that is, hit brew edit protobuf.)

Per https://golang.org/wiki/InvalidFlag, does it build with
CGO_LDFLAGS_ALLOW=-D_THREAD_SAFE set in the environment?

Ah, I missed those docs. Thanks!

Yep, it builds with that env var set.

Basically, we don't expect pkg-config --libs to contain a -D option. There's no obvious reason for the option, since it won't affect linking. If it's fixed on protobuf master then I don't think there is anything to do here. The older package is usable with the environment variable.