[WIP] update Gaudi to v35r0 : cmake overhaul

Use the new cmake build system for Gaudi

Charles, this is a completely useless MR... I created !37942 (closed) exactly to demonstrate what switching to the new Gaudi CMake configuration will look like. As you can see in that MR, the version of Gaudi used is not part of this repository anymore. At least not in the way as it is now.

With the new v35r0.000 tag in place, tomorrow I will:

• Update atlasexternals!699 (merged) to use that tag.
  • Probably updating the code to allow setting the exact Gaudi version on the command line. To make it possible to choose the Gaudi version similar to the LCG version in the build_externals.sh scripts.
• Once that MR is in the master branch of atlas/atlasexternals (hopefully sometime tomorrow), I'll create a new MR based on !37942 (closed) that will update this repository to the new tag of atlas/atlasexternals.

But this MR will not play a role in any of this...

Argh - I had forgotten that the way of selecting the Gaudi version was going to change. duh. I will close this.

closed