C++ Flags Fix, main branch (2023.10.17.)
GeoModel is the next thing to fix for ATEAM-905, after a number of other CMake fixes. (Most recently in https://github.com/lwtnn/lwtnn/pull/174, for exactly this type of issue.)
The story is long, but the fix in this MR is relatively simple. One should never outright overwrite CMAKE_CXX_FLAGS in a project's code, as that prevents clients from inserting their own flags into the build. Like what we do in https://bigpanda.cern.ch/nview/?nightly=main--archflagtest_Athena_x86_64-centos7-gcc11-opt to add -march=x86-64-v3 -ftree-vectorize -fvect-cost-model=very-cheap to our build.