use Debian packages gtest.

Here is an example:

    http://anonscm.debian.org/cgit/debian-science/packages/gazebo.git/tree/debian/patches/0002_use_system_gtest.patch

Another helpful thing might be

    https://github.com/PointCloudLibrary/pcl/blob/master/cmake/Modules/FindGtest.cmake


See https://lists.debian.org/debian-science/2015/08/msg00014.html ff.
