CMake example does not build tests

I created a CMake project by copy-pasting the example CMakeLists.txt

I cloned abseil-cpp, cctz and googletest into subdirectories, created a build directory and ran `cmake ..` there.

```

Re-run cmake no build system arguments

-- The C compiler identification is Clang 7.0.0

-- The CXX compiler identification is Clang 7.0.0

-- Check for working C compiler: /usr/local/bin/clang

-- Check for working C compiler: /usr/local/bin/clang -- works

-- Detecting C compiler ABI info

-- Detecting C compiler ABI info - done

-- Detecting C compile features

-- Detecting C compile features - done

-- Check for working CXX compiler: ~/bin/clang++6

-- Check for working CXX compiler: ~/bin/clang++6 -- works

-- Detecting CXX compiler ABI info

-- Detecting CXX compiler ABI info - done

-- Detecting CXX compile features

-- Detecting CXX compile features - done

-- Found PythonInterp: /usr/bin/python (found version "2.7.14") 

-- Looking for pthread.h

-- Looking for pthread.h - found

-- Looking for pthread_create

-- Looking for pthread_create - not found

-- Check if compiler accepts -pthread

-- Check if compiler accepts -pthread - yes

-- Found Threads: TRUE  

CMake Warning at cctz/CMakeLists.txt:17 (find_package):

  By not providing "Findbenchmark.cmake" in CMAKE_MODULE_PATH this project

  has asked CMake to find a package configuration file provided by

  "benchmark", but CMake did not find one.



  Could not find a package configuration file provided by "benchmark" with

  any of the following names:



    benchmarkConfig.cmake

    benchmark-config.cmake



  Add the installation prefix of "benchmark" to CMAKE_PREFIX_PATH or set

  "benchmark_DIR" to a directory containing one of the above files.  If

  "benchmark" provides a separate development package or SDK, be sure it has

  been installed.





-- Could NOT find GTest (missing: GTEST_LIBRARY GTEST_INCLUDE_DIR GTEST_MAIN_LIBRARY) 

-- Could NOT find GMock (missing: GMOCK_LIBRARY GMOCK_MAIN_LIBRARY GMOCK_INCLUDE_DIR GTEST_FOUND) 

-- Could NOT find GTest (missing: GTEST_LIBRARY GTEST_INCLUDE_DIR GTEST_MAIN_LIBRARY) 

-- The following OPTIONAL packages have been found:



 * PythonInterp



-- The following REQUIRED packages have been found:



 * Threads, the system thread library



-- The following REQUIRED packages have not been found:



 * benchmark, a microbenchmark support library, <https://github.com/google/benchmark>

 * GMock, the Google C++ mocking framework, <https://github.com/google/googletest>

 * GTest, the Google C++ test framework, <https://github.com/google/googletest>



CMake Error at /usr/share/cmake-3.9/Modules/FeatureSummary.cmake:459 (message):

  feature_summary() Error: REQUIRED package(s) are missing, aborting CMake

  run.

Call Stack (most recent call first):

  cctz/CMakeLists.txt:140 (feature_summary)





-- Configuring incomplete, errors occurred!

```



Abseil seems to be unable to find googletest.

benchmark is not mentioned at all in the readme.

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