message: external_example: trying to get the includes right
diff: +cmake_minimum_required(VERSION 3.1)
  +
  add_executable(external_example_integration_tests_runner
  - ${PROJECT_SOURCE_DIR}/core/unittests_main.cpp
  + ../../../core/unittests_main.cpp
  hello_world.cpp
  )
  include_directories(
  - ${PROJECT_SOURCE_DIR}/core
  - ${PROJECT_SOURCE_DIR}/external_example
  + ../../core
  + ../../integration_tests
  + ../../external_example
  + ../../third_party/gtest/googletest/include
  + ../../third_party/gtest/googlemock/include
  )
  target_link_libraries(external_example_integration_tests_runner
  