Add dependency manager

[artiomn]

Currently only SIMDJSON is used as a submodule. But in the future there may be other dependencies (for example, a third-party HTTP server or exclude getopt from the codebase).
Submodules are very inconvenient, not very effective, and tie the project to a version control system, which should not be the case.

Probably better solution will be use dependency manager. CPM is lightweight and written in pure CMake.

[rdementi]

a good idea. Will try it when we will have more dependencies to justify it and improve ROI

[artiomn]

Besides CPM, there are Hunter and Conan.io .
But Conan.io in my opinion is not very convenient.
I didn't use Hunter, probably it's more flexible than CPM, but CPM written in pure CMake and mostly satisfies my requirements.

[artiomn]

@rdementi, @opcm, which would you prefer to use?

[artiomn]

I can try to integrate CPM, if you want.

[ogbrugge-work]

@artiomn rdementi is supposed to be on vacation this week, don't make him work so hard ;-)

[artiomn]

@artiomn rdementi is supposed to be on vacation this week, don't make him work so hard ;-)

Ok, I'm waiting. :-)

[rdementi]

Thanks for volunteering!

I discussed it with @twillhal . Adding cmake/CPM.cmake would potentially require an internal review and approval. We will do it but unfortunately I can't give a timeline.

[artiomn]

I can do this, if you approve this change.

[rdementi]

adding CPM.cmake has been approved. It is now in the master branch: https://github.com/intel/pcm/blob/master/cmake/CPM.cmake