Add CMake build support

[guitargeek]

Description:

This PR adds the relevant files to build combine with CMake. I had this around privately for a long time, and I think it would be good to expose this also to other users and developers that want to build combine with CMake.

It is a draft PR for now because there is still something I want to improve: right now, the python libraries are always installed in the system Python package directory, but it would be good to have a configuration option maybe called PYTHON_LOCAL to install to the local library directory for the user (like --local for pip). I will implement this when I have time, but I still wanted to open the PR already to get feedback from interested people that want to give it a shot.

You can try it out with:

git clone [email protected]:guitargeek/HiggsAnalysis-CombinedLimit.git -b 112x_cmake HiggsAnalysis/CombinedLimit
cd HiggsAnalysis/CombinedLimit
mkdir build
cd build
cmake3 -DCMAKE_INSTALL_PREFIX:PATH=/usr ..
cmake3 --build . -j8
make install

Unlike the standalone Makefile that is already in the repo, this should work not only on lxplus because of the smart CMake macros that try to find the dependencies at the right place.

Tested on:

I'll try to make it work for the "vanilla" versions of the operating systems, so no cmsenv allowed for example. A yellow sign means it doesn't work immediately out of the box, but after only small adaptations to the environment like sourcing a newer ROOT version.

• CentOS 7 (lxplus7) ❌
  • compiler is too old
• CentOS 8 (lxplus8) ➕
  • requires ROOT 6.26.06 sourced from cvmfs
• Arch Linux ✔️
  • package can be generated with this PKGBUILD file

Still todo:

• add PYTHON_LOCAL option described before
• automatically detect correct C++ standard matching the ROOT installation
• remove restriction that source code directory must be named HiggsAnalysis/CombinedLimit DONE
• try to get rid of using cmake_path because it's only available in cmake 3.20 which is quite new

[guitargeek]

Superseded by: #943.

Thanks @will-cern!