This change improves spirv-fuzz CMake code to be more compatible with other projects that might want to include spirv-fuzz as a sub-project.
* Add a CMake option for building spirv-fuzz.
* We now check if protobuf targets are already available.
* We no longer specify `-DGOOGLE_PROTOBUF_NO_RTTI -DGOOGLE_PROTOBUF_USE_UNALIGNED=0`; a newer version of protobuf does not require this. Note that we probably should have specified this for protobuf targets as well, but this is no longer needed.
* Updated protobuf version in Kokoro scripts and README.md.
