need for flag for incompatible-changes

[Joe Buck]

> I wonder about the comment that even GNU ld often would only be able
> to say "sorry, incompatible object files".  Some things, like naming
> (namespace support) clearly aren't like that.

We not only plan to add namespace support to the compiler, but we
then need to redo libstdc++ to put the whole standard library in the
std namespace.  That's what breaks compatibility, and it doesn't show
up at the level of object file version -- it is a change in the API.
Adding namespaces wouldn't break compatibility if namespaces are not
used.

The other major change that needs to be done to C++ is to redo iostreams
as specified by the draft standard (make iostream a template).  This
risks breaking libc compatibility on Linux if it is not done carefully.