Fix build when included with cmake_add_subdirectory

[rotu]

Currently breaks when built as a sub-project. This fixes that.

[rotu]

still broken:

-- Build files have been written to: /tmp/cmake-build-eloquent-debug/src/osrf/osrf_testing_tools_cpp/osrf_testing_tools_cpp/googletest-1.8.0-extracted
[100%] Built target googletest
-- Found PythonInterp: /usr/bin/python3 (found version "3.6.8") 
CMake Error at /tmp/cmake-build-eloquent-debug/src/osrf/osrf_testing_tools_cpp/osrf_testing_tools_cpp/googletest-1.8.0-extracted/googletest-1.8.0-src/googletest/cmake/internal_utils.cmake:149 (add_library):
  add_library cannot create target "gtest" because another target with the
  same name already exists.  The existing target is a static library created
  in source directory "/opt/ros/master/install/src/gtest_vendor".  See
  documentation for policy CMP0002 for more details.
Call Stack (most recent call first):
  /tmp/cmake-build-eloquent-debug/src/osrf/osrf_testing_tools_cpp/osrf_testing_tools_cpp/googletest-1.8.0-extracted/googletest-1.8.0-src/googletest/cmake/internal_utils.cmake:172 (cxx_library_with_type)
  /tmp/cmake-build-eloquent-debug/src/osrf/osrf_testing_tools_cpp/osrf_testing_tools_cpp/googletest-1.8.0-extracted/googletest-1.8.0-src/googletest/CMakeLists.txt:90 (cxx_library)


CMake Error at /tmp/cmake-build-eloquent-debug/src/osrf/osrf_testing_tools_cpp/osrf_testing_tools_cpp/googletest-1.8.0-extracted/googletest-1.8.0-src/googletest/cmake/internal_utils.cmake:149 (add_library):
  add_library cannot create target "gtest_main" because another target with
  the same name already exists.  The existing target is a static library
  created in source directory "/opt/ros/master/install/src/gtest_vendor".
  See documentation for policy CMP0002 for more details.
Call Stack (most recent call first):
  /tmp/cmake-build-eloquent-debug/src/osrf/osrf_testing_tools_cpp/osrf_testing_tools_cpp/googletest-1.8.0-extracted/googletest-1.8.0-src/googletest/cmake/internal_utils.cmake:172 (cxx_library_with_type)
  /tmp/cmake-build-eloquent-debug/src/osrf/osrf_testing_tools_cpp/osrf_testing_tools_cpp/googletest-1.8.0-extracted/googletest-1.8.0-src/googletest/CMakeLists.txt:91 (cxx_library)


CMake Error at /tmp/cmake-build-eloquent-debug/src/osrf/osrf_testing_tools_cpp/osrf_testing_tools_cpp/googletest-1.8.0-extracted/googletest-1.8.0-src/googletest/CMakeLists.txt:92 (target_link_libraries):
  Attempt to add link library "gtest" to target "gtest_main" which is not
  built in this directory.

  This is allowed only when policy CMP0079 is set to NEW.

[rotu]

@nuclearsandwich why do both both this and https://github.com/ament/googletest provide googletest?

[nuclearsandwich]

@nuclearsandwich why do both both this and ament/googletest provide googletest?

I haven't the foggiest idea.

Working backwards from what I do know, I don't believe this package "provides" googletest but rather keeps its own copy as this is a pure CMake package not an ament_cmake package and so does not use the ament set of packages for testing. A quick browse through the package CMake code suggests that this is how googletest upstream recommends that source be included into consuming projects.

[wjwwood]

@rotu this package was meant to be used outside of ROS 2 as well as within it, and therefore doesn't depend on ament or the ament mechanisms for googletest. It would be possible for ament to use this, but it's currently not the case.

[wjwwood]

I just came across this when cleaning up some other issues, I'll see about incorporating this in the next release.

[rotu]

Thanks, @wjwwood. For your convenience, I've resolved merge conflicts.

[wjwwood]

@rotu sorry for the delay, but I had one question after re-reviewing it.

[wjwwood]

Why is this needed? I assume there was a duplicate target, but honestly I'm not sure this is the right way to handle it. If the target already exists then either osrf_testing_tools_cpp_require_googletest() was called more than once (which is explicitly not allowed) or it was created by another piece of CMake logic, which is also bad, since it might not have the version osrf_testing_tools_cpp_require_googletest() would have given.