preliminary libstdc++ ABI docs

[Benjamin Kosnik]

> > If you see symbols in the resulting output with "GLIBCPP_3.x" as
> > part of the name, then the executable is versioned. Here's an
> > example:
> > 
> >          U _ZNSt8ios_base4InitC1Ev@@GLIBCPP_3.1
> 
> Should that ABI version number be changed on the trunk?

I'm not quite sure. I'm looking into this.

> Should that number have any relation to configure.in's
> libtool_VERSION, which also tracks the ABI?  (Everyone remember that
> the "version" in the map file is just a text string and does not need
> to be able to be "calculated" like a libtool version would, or any
> other version for that matter.  I would think that "3.1" should be
> either "3.2" or "5".)

Looking at libgcc_s, as well as glibc, it looks like 3.2.

> Should a new GLIBCPP_foo inherit the symbols from GLIBCPP_3.1?  I
> think it's supposed to work that way, but this breaks new ground for
> me.

Not quite sure what you are getting at here.

-benjamin