need for flag for incompatible-changes

[Joe Buck]

> If object files contained version numbers, you wouldn't have to break
> anything.  (More precisely, you would only have to break things that
> are older than the oldest version you want to continue supporting in
> the linker.)

Where we have control over the linker (Linux, or platforms where GNU ld
will work), we could use the linker to handle some types of
incompatibilities.  In many cases, I'm afraid that all it will lets
us do is say "sorry, incompatible object files".