benchmark

If gtest targets are already defined, use them. (#777)

This allows a project to include both googletest and benchmark as top-level git submodule.

This allows incorporating Benchmark to an existing CMake project that already incorporates googletest.
https://github.com/google/googletest/blob/master/googletest/README.md#incorporating-into-an-existing-cmake-project
https://github.com/abseil/abseil-cpp/tree/master/CMake#incorporating-abseil-into-a-cmake-project