https://github.com/google/benchmark/pull/801 is stuck with some cryptic cmake failure due to
some linking issue between googletest and threading libraries.

I suspect that is mostly happening because of the, uhm,
intentionally extremely twisted-in-the-brains approach that is being used to
actually build the library as part of the buiild,
except without actually building it as part of the build.

If we do actually build it as part of the build,
then all the transitive dependencies should magically be in order,
and maybe everything will just work.

This new version of cmake magic was written by me in
https://github.com/darktable-org/rawspeed/blob/0e22f085c57cd629983c7351c598abd744b421b5/cmake/Modules/GoogleTest.cmake.in
https://github.com/darktable-org/rawspeed/blob/0e22f085c57cd629983c7351c598abd744b421b5/cmake/Modules/GoogleTest.cmake, based on the official googletest docs and LOTS of experimentation.