versioning C++ includes

[Benjamin Kosnik]

> This is out of our control because distributors put their libraries in
> particular places specified at configure time, that aren't necessarily
> writable by developpers.  At any rate, it isn't something in the scope
> of V3.  We can publish some guidance about our ABI (that is what
> Benjamin plans to do) but that is all about it.

Right. Although Nathan has a real issue, it's out of the scope of
libstdc++-v3 per se, and in the realm of particular distributions. Many
use compatibility libraries to deal with this, for instance.

In general, libstd++-v3 is assumed to be compiled with the ABI of the
g++ that is shipped with it. On stable branches, this means that the
compiler ABI is assumed stable. Doing more complex setups requires
additional effort, which I am not attempting to standardize.

This patch is in now.

-benjamin