cmake_minimum_required version affects rpath linking on macOS

[vvolkl]

Opening this issue here as it affects a number of ilcsoft packages: When requiring only cmake version >= 2.8, policy CMP0042 is set to OLD and MACOSX_RPATH is disabled unless explictly enabled, which may lead to issues and confusion, especially when building with spack, which relies on rpath linking.

Setting this policy to NEW would be helpful, but a number of packages depend on ROOT, which nowadays requires CMake 3.16 (3.19 on macOS) anyway. The question is thus if for those packages the cmake_minimum_required version can be bumped or compatibility with older version of ROOT is necessary ( ROOT started requiring cmake v3 in 6-08-00: root-project/root@e99a0e0).