Unify file description in C++ comment and help displayed at runtime

This is done to keep them in sync. Actually they were already
slightly diverging.
Clarify with a comment the hardcoded presence of gcc46
Extract platform defines addition to cling args into a function
New main to switch between rootcling and reflex accorting to exe name

This allowed to encapsulate the old main in a function and start
fresh for the implementation of the "new" genreflex functionalities.
General cleanup: fcns names, unused code
Clean R__ from structs as well