changes for build modification and protoc-plugin

[kumarakshays]

fixes-#1449

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[sampajano]

Thanks for the change, @kumarakshays!

This is on the right track!!

Although, one error we need to address is following:

ERROR - [JSC_READ_ERROR] Cannot read file ../../third_party/protobuf/js: ../../third_party/protobuf/js

This is because javascript has been removed from newer protobuf repo, so we need to migrate our protobuf JS dependency to https://github.com/protocolbuffers/protobuf-javascript too. (see #1449 (comment))

However we can probably find a way to decouple that by keeping the JS dependency (e.g. rename it to /thirdparty/protobuf-old/ and keep the 3.15.6 version) and fix the c++ compilation first using this change.

[kumarakshays]

Thank you,@sampajano
Once the C++ compilation is fixed, I can create a follow-up PR to migrate the JS dependency to protobuf-javascript.