[cmake] Allow to populate ROOT using FetchContent

[flagarde]

This Pull request:

Allow to populate ROOT using FetchContent. Only compilation has been tested and further work should be done to allow to used the fetched ROOT.

snippet:

cmake_minimum_required(VERSION 3.16)

project(test)

set(FETCHCONTENT_QUIET false)
include(FetchContent)
FetchContent_Declare(MyRoot GIT_REPOSITORY https://github.com/flagarde/root.git GIT_TAG FetchContent)
#FetchContent_MakeAvailable(MyRoot)
# Check if population has already been performed
FetchContent_GetProperties(MyRoot)
if(NOT myroot_POPULATED)
  # Fetch the content using previously declared details
  FetchContent_Populate(MyRoot)

  # Set custom variables, policies, etc.
  # ...

  # Bring the populated content into the build
  add_subdirectory(${myroot_SOURCE_DIR} ${myroot_BINARY_DIR})
endif()

Changes or fixes:

Mainly some CMake variables.

Checklist:

• tested changes locally
• updated the docs (if necessary)

This PR fixes :

It's a rework on #9516 . This PR should not change anything for users compiling ROOT as standalone project. However until now, it is not possible to directly include ROOT in an other project using FetchContent. This PR is focusing on the CMake configuration, generation steps and in the compilation process. No work has been done yet to let upstream project to find ROOT and to use it. I made a try on the last point but I think it is better to split the task in more steps.

Usage of FetchContent is more and more used to create "standalone" project downloading and compiling all its dependencies. In the other PR tries some people and official maintainers have raised their interest on this feature.

[phsft-bot]

Can one of the admins verify this patch?

[dpiparo]

Thanks for this code. Could you please add in the description of the PR why you are doing this, and especially what problem you are solving? A good way to tackle this would be to try to convince the reader that thanks to the code, ROOT will be like before (e.g. nothing breaks for the experiments, users, in the development process of ROOT iteself) but better (e.g. there is a reduction in the cost of maintenance, development, testing, or an increase of runtime performance).

[flagarde]

Sure,I tried to explain a bit better this PR

[dpiparo]

Thanks. Could you point the reader to projects in need of this upgrade in the PR description?

[flagarde]

@dpiparo Well, we had some programs we used internally in my university group and I did some work to allow to FetchContent ROOT directly for this project. Then when I deleted my own ROOT repo and the last PR disappered people told me they could be still interested (#9516). This is the reason I submited a PR again.