Unnamed histograms generate wrong C code in SavePrimitve(). Of course
an unnamed histogram doesn’t make real sense but some people (like in ROOT-5932)
seem to like them. It is protected now.
Missing include for UsedAttr
Need virtual d’tors (XCode warning)
Fix -fexceptions for CMake.
Follow current clang/llvm Option syntax (late fix from llvm/clang upgrade).