CMake: behave as before if BUILD_SHARED_LIBS is not defined. Needed for examples and DD4hep external packageS

CMake: add settings DD4HEP_BUILD_PACKAGES to chose which packages to build, DD4HEP_BUILD_EXAMPLES to build examples

it will be the responsible of the user to chose a working combination and order of packages
Use DD4hep::<Package> so that cmake will fail if required package was not enabled