Build of Key4hep software on MacOS

[vvolkl]

Keeping track of the progress building the software stack on macos (I'm using a build machine running catalina / apple-clang 12.0.0):

• When I started out, the system python could not be used to run spack. Has since been fixed in upstream spack.
• Since some packages have some fortran codes, a fortran compiler is needed, which is not provided by xcode / clang. The easiest way I found is to install gcc with spack and add gfortran to the appleclang entry in compilers.yaml
• Openblas can be built with spack since october, provided gfortran from gcc@10: is used .
• I nevertheless use veclibfort to provide blas
• ROOT cannot be built with +r because spack doesn't correctly handle the java dependency of r on macos
• I manage to build root with +aqua (enables cocoa) and ~x~opengl. The opengl dependencies in the root recipe definitely need to be updated in the root recipe. maybe with corrected dependencies I can enable those options as well.
• Some packages (kaltest, kaldet) need eve which is not included in my current working build
• issue building gaudi: https://gitlab.cern.ch/gaudi/Gaudi/-/issues/158