CxxUtils: enable cppcheck by default
As discussed during the SW Weekly meeting on Sept 26th and requested on ATLINFR-5443, this MR enables cppcheck by default for builds in the main branch (full builds and WorkDir builds).
Also emit an explicit warning in case cppcheck was requested to be ON but cannot be found. As cppcheck is included in our current LCG layer, no additional setup is needed to run it.
Edited by Frank Winklmeier