Adding Google benchmarking library.

third_party/google_benchmark/src/CMakeLists.txt

+# Allow the source files to find headers in src/
+include_directories(${PROJECT_SOURCE_DIR}/src)
+
+if (DEFINED BENCHMARK_CXX_LINKER_FLAGS)
+  list(APPEND CMAKE_SHARED_LINKER_FLAGS ${BENCHMARK_CXX_LINKER_FLAGS})
+  list(APPEND CMAKE_MODULE_LINKER_FLAGS ${BENCHMARK_CXX_LINKER_FLAGS})
+endif()
+
+file(GLOB
+  SOURCE_FILES
+    *.cc
+    ${PROJECT_SOURCE_DIR}/include/benchmark/*.h
+    ${CMAKE_CURRENT_SOURCE_DIR}/*.h)
+
+add_library(benchmark ${SOURCE_FILES})
+set_target_properties(benchmark PROPERTIES
+  OUTPUT_NAME "benchmark"
+  VERSION ${GENERIC_LIB_VERSION}
+  SOVERSION ${GENERIC_LIB_SOVERSION}
+)
+
+# Link threads.
+target_link_libraries(benchmark  ${BENCHMARK_CXX_LIBRARIES} ${CMAKE_THREAD_LIBS_INIT})
+find_library(LIBRT rt)
+if(LIBRT)
+  target_link_libraries(benchmark ${LIBRT})
+endif()
+
+# We need extra libraries on Windows
+if(${CMAKE_SYSTEM_NAME} MATCHES "Windows")
+  target_link_libraries(benchmark Shlwapi)
+endif()
+
+set(include_install_dir "include")
+set(lib_install_dir "lib/")
+set(bin_install_dir "bin/")
+set(config_install_dir "lib/cmake/${PROJECT_NAME}")
+
+set(generated_dir "${CMAKE_CURRENT_BINARY_DIR}/generated")
+
+set(version_config "${generated_dir}/${PROJECT_NAME}ConfigVersion.cmake")
+set(project_config "${generated_dir}/${PROJECT_NAME}Config.cmake")
+set(targets_export_name "${PROJECT_NAME}Targets")
+
+set(namespace "${PROJECT_NAME}::")
+
+include(CMakePackageConfigHelpers)
+write_basic_package_version_file(
+    "${version_config}" VERSION ${GIT_VERSION} COMPATIBILITY SameMajorVersion
+)
+
+configure_file("${PROJECT_SOURCE_DIR}/cmake/Config.cmake.in" "${project_config}" @ONLY)
+
+# Install target (will install the library to specified CMAKE_INSTALL_PREFIX variable)
+install(
+  TARGETS benchmark
+  EXPORT ${targets_export_name}
+  ARCHIVE DESTINATION ${lib_install_dir}
+  LIBRARY DESTINATION ${lib_install_dir}
+  RUNTIME DESTINATION ${bin_install_dir}
+  INCLUDES DESTINATION ${include_install_dir})
+
+install(
+  DIRECTORY "${PROJECT_SOURCE_DIR}/include/benchmark"
+  DESTINATION ${include_install_dir}
+  FILES_MATCHING PATTERN "*.*h")
+
+install(
+    FILES "${project_config}" "${version_config}"
+    DESTINATION "${config_install_dir}")
+
+install(
+    EXPORT "${targets_export_name}"
+    NAMESPACE "${namespace}"
+    DESTINATION "${config_install_dir}")