[benchmark] Deploy Celero and Benchmark as external project instead of Conan

From: Mateusz Loskot (mateusz_at_[hidden])
Date: 2019-06-19 11:37:55

Folks,

I have re-worked [1] CMake configuration for the benchmarks prototype at

https://github.com/boost-gil/benchmark-prototype/tree/benchmark

For some reason, possibly due to a mistake on my side, I had to run
`git push --force`, so please `git branch -D benchmark` your local branch
and fetch and checkout the fresh update.

Sorry for inconvenience.

[1] https://github.com/boost-gil/benchmark-prototype/commit/bd00c15fd65d4e973c91f0c5eaa1cdac6e1e6fce

Best regards,
-- 
Mateusz Loskot, http://mateusz.loskot.net