Makefile: Fix protoc compile option to disable building tests
The Protobuf project has seemingly renamed their CMake options to all
have a protobuf_
prefix. While the deault options we set still work,
this broke the -DBUILD_TESTS=OFF
switch.
Fix this and adapt accordingly to significantly reduce build times of this project. On my machine, it decreases from about 58.9s to 22.8s.