Use latest Darwin with FetchContent
This also includes a bunch of changes to sync with latest Darwin, for instance using Darwin's create_version_header() CMake helper instead of duplicating part of the Darwin code.
$ source /cvmfs/sft.cern.ch/lcg/views/LCG_107a/x86_64-el9-gcc14-opt/setup.sh
$ cmake -B build
$ cmake --build build --target install
# works
Supersedes !1 (closed). I confirmed that Darwin from the environment gets picked up when present (used one I pulled from the Installer
repo).