Add a CMakeLists.txt for compiling with cmake

[will-cern]

This is tested by compiling on top of the StatAnalysis releases (setup instructions in comments in the CMakeLists.txt)

There will be conflicts for some classes which are compiled by both combine and RooFitExtensions (the ATLAS extra-classes library), which we should ultimately decide how to resolve (push common classes into RooFit, is best option).

[guitargeek]

I checked that this also compiling on lxplus9 after doing:

source /cvmfs/sft.cern.ch/lcg/views/LCG_105/x86_64-el9-gcc12-opt/setup.sh

Nice!

[anigamova]

closing, merged #967 instead