CMake: Replace ExternalProject_Add with FetchContent

[meastp]

This reduces the amount of code for including/building dependencies. The only drawback is an increase in the CMake version required.

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[googlebot]

CLAs look good, thanks!

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[bogdandrutu]

@g-easy this looks good to me (from a person with 0 knowledge about cmake, but I like when code gets deleted). What do you think?

[meastp]

We already discussed it in #244, but I'd like @g-easy to look at it anyway :)

[g-easy]

Please make this pass on Travis. :)

[meastp]

Well, I can't run the tools/format.sh script as I'm on Windows. Could you help me with that like last time? :)

The other failure is simply because the travis node's CMake version is too old:

CMake Error at CMakeLists.txt:15 (cmake_minimum_required):
CMake 3.11 or higher is required. You are running version 3.9.2

IS trusty in the travisci-config Ubuntu Trusty 14.04 ? That's a pretty old distro... Why aren't you using xenial?

[bogdandrutu]

@meastp you can create a small docker image map your clone directory, and run the script :) maybe we should have a "tools/format_docker.sh" which actually is useful not only for your usecase but also to make sure that we run that specific clang-format.

[meastp]

@meastp you can create a small docker image map your clone directory, and run the script :) maybe we should have a "tools/format_docker.sh" which actually is useful not only for your usecase but also to make sure that we run that specific clang-format.

What if travis could just push a commit to the pull request with the required changes if the tools/format.sh fails? :)

https://gist.github.com/willprice/e07efd73fb7f13f917ea

[g-easy]

Regular Xenial ships cmake 3.5.1. I got this running locally with ubuntu:cosmic. https://launchpad.net/ubuntu/+source/cmake

[g-easy]

@coryan Is the cmake 3.11 requirement okay with you?

[coryan]

And Ubuntu:bionic is 3.10, and let's not get started on CentOS. I think 3.11 might be too high given the current versions on popular distros.

[meastp]

@coryan What about including FetchContent.cmake directly, then, as was suggested in #256 (comment)? It is not much code, the opencensus-cpp cmake-code is cleaner, and in a while when the popular distros get CMake >= 3.11, we can just delete FetchContent.cmake and FetchContent.cmake.in.

[g-easy]

I think FetchContent.cmake is worth a try. What is the minimum version requirement with this option?

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We found a Contributor License Agreement for you (the sender of this pull request), but were unable to find agreements for all the commit author(s) or Co-authors. If you authored these, maybe you used a different email address in the git commits than was used to sign the CLA (login here to double check)? If these were authored by someone else, then they will need to sign a CLA as well, and confirm that they're okay with these being contributed to Google.
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[googlebot]

CLAs look good, thanks!

ℹ️ Googlers: Go here for more info.

[meastp]

@coryan @g-easy It looks like this works with the old cmake versions. :)

@g-easy Could you please fix format.sh for me?

[g-easy]

Nice! Thanks!