Unknown CMake command "protobuf_generate_cpp".

I install all the prerequisites. But ninja check failed
So I just want to build a simple demo, then it comes the flowing errors.
My Linux version : ubuntu 18.04
libprotoc version: 3.0.0
clang version: 6.0.0-1ubuntu2
cmake version: 3.10.2
```
raven@ubuntu:~/libprotobuf-mutator/examples/libfuzzer/build$ cmake ..
CMake Error at CMakeLists.txt:25 (protobuf_generate_cpp):
Unknown CMake command "protobuf_generate_cpp".

CMake Warning (dev) in CMakeLists.txt:
No cmake_minimum_required command is present. A line of code such as

cmake_minimum_required(VERSION 3.10)

should be added at the top of the file. The version specified may be lower
if you wish to support older CMake versions for this project. For more
information run "cmake --help-policy CMP0000".
This warning is for project developers. Use -Wno-dev to suppress it.

-- Configuring incomplete, errors occurred!
See also "/home/raven/libprotobuf-mutator/examples/libfuzzer/build/CMakeFiles/CMakeOutput.log".

```