message: external_example: fix paths
  This fixes the paths for me in a try according to:
diff: cmake_minimum_required(VERSION 3.1)
  add_executable(external_example_integration_tests_runner
  - ../../core/unittests_main.cpp
  + ${CMAKE_SOURCE_DIR}/core/unittests_main.cpp
  hello_world.cpp
  )
  include_directories(
  - ../../core
  - ../../integration_tests
  - ../../external_example
  - ../../third_party/gtest/googletest/include
  - ../../third_party/gtest/googlemock/include
  + ${CMAKE_SOURCE_DIR}/core
  + ${CMAKE_SOURCE_DIR}/integration_tests
  + ${CMAKE_SOURCE_DIR}/third_party/gtest/googletest/include
  + ${CMAKE_SOURCE_DIR}/third_party/gtest/googlemock/include
  )
  target_link_libraries(external_example_integration_tests_runner
  