Tentative build of art_{Version,Utilities} libraries

Recurse into art subdirectory and hence Version and Utilities
subdirectories. Replace cet builds with standard CMake commands,
using new INTERFACE_XXX functionality to provide linking and
relevant includes. Do not install libs/headers as yet.

Update top level CMake script to find Reflex component of ROOT
needed by art_Utilities.

NB: find_package for ROOT relies on existence of "ROOTConfig"
file, which means the install of ROOT must have been built with
CMake. However, it appears that the installed ROOTConfig does not
function correctly as it encodes paths incorrectly, at least for
the "gnuinstall" style.