Build ok since pr #4898 is merged but extension dosn't load (since 7.3.0alpha4, was OK with alpha3)
$ php --no-php-ini --define extension=modules/protobuf.so --modules | grep protobuf
Output is empty. No error message.
P.S. as I can't sign your CLA, I don't plan to work on this issue to propose a fix, so, do it yourself.