[Rd] CRAN: Update protocol buffers on macOS? (for RProtoBuf)

[Jonathon Love]

Hi,

I notice that the CRAN binary for the macOS version of RProtoBuf is 
built against quite an old version of protocol buffers (from 2014, 
before v3 format support was added).

the windows version is (blessedly) kept up-to-date, but I'd like to 
float the suggestion that the macOS version get an update too.

with thanks

jonathon