4.2.1 abi compatibility with 4.0.0

[Benjamin Kosnik]

> > I don't believe that mac os has symbol versioning or ABI checking.
> 
> In 4.2.1 we use config/abi/pre/gnu.ver to build up the library.

Really? That's interesting. I don't have access to a darwin 9
platform, but I am surprised to hear this.

I thought darwin/MachO used an incompatible mechanism from ELF symbol
versioning, one that only allowed two versions at a time.
 
> > I assume that Apple is doing such things themselves.
> 
> We build and install gcc, and shipped the 4.0.0 libstdc++ as a
> shared library, in the past.  We're now updating the library to 4.2.1
> and things mostly work, though, this symbol went missing.

Exported symbols are checked against a known good list, so I'm assuming
we would have seen this. (That said, I've not looked in detail at this.)

Do the darwin exports (from baseline_symbols in the build/testsuite
directory) match linux then?

If not, you might consider adding support for ABI checking, and start by
providing baselines for darwin builds.

-benjamin