gitian: Make protobuf win32 intermediate output deterministic

While building protobuf in different environments we noticed that the host tool protoc was slightly different between builds (a symbol table sorting issue).
Add a deterministic seed for gcc as well as disable zlib support just in case.

Exected output is now:

    e2e403e1a08869c7eed4d4293bce13d51ec6a63592918b90ae215a0eceb44cb4 protobuf-win32-2.5.0-gitian-r4.zip
    a0999037e8b0ef9ade13efd88fee261ba401f5ca910068b7e0cd3262ba667db0 protobuf-win64-2.5.0-gitian-r4.zip

No effect on final executables so no version bump.