Add benchmark as cm_generate_makefile option

[brian-kelley]

@cz4rs @lucbv

It would be nice if cm_generate_makefile had a flag like --with-benchmark to set -DKokkosKernels_ENABLE_BENCHMARK=ON.

[lucbv]

Yeah, I have been purposely refraining from doing it just yet as we are still making some heavy changes and I do not want folks to mistakenly believe that because the option is in place it is fully ready... I believe we are getting pretty close to that point though.