update to googletest 1.8

[dirk-thomas]

The headers provided by gtest/gmock_vendor are now preferred over the system headers since we want to use the newer version of googletest coming with the vendor packages.

The main library needs to be linked before the non-main library since the first one might use symbols which are defined int he second one.

The gmock_vendor doesn't contain its own version of gtest anymore but uses the headers provided by gtest_vendor instead. That also removes the need for the extension point.

Required for ros2/ros2#385.

[mikaelarguedas]

this looks good to me, a further improvement would be to get rid completely of gmock_vendor references given that the ament_cmake_gmock provides the necessary wrapper so that none of our packages have to find the "vendor" packages explicitly.

As a side note for the future: starting with zesty, a dependency-free googletest debian package is available

[dirk-thomas]

a further improvement would be to get rid completely of gmock_vendor references given that the ament_cmake_gmock provides the necessary wrapper so that none of our packages have to find the "vendor" packages explicitly.

The packages gmock_vendor / gtest_vendor could be renamed anytime to googlemock / googletest. I tried to keep this round of PRs as simple as possible and therefore didn't do it.

The packages still need to exist though until we have googletest as a binary package on all supported platforms.