As noticed by Jonathan, the namespace chosen for this extension is not
in the implementation namespace and clashes with a similar name in
<functional>: the placeholders for bind.

Fixed thusly. And another name injection patchlet.

However, there's a bit of an added wrinkle in that now libstdc++ is
linked in earlier in the build, and is not just this target library.
So, to avoid confusion and build errors, 

--enable-symvers=gnu-versioned-namespace

Now sets libstdc++.so's SONAME to libstdc++.so.7, not
libstdc++.so.6.x.x. Ie, incompatible. 

This is not really a huge conceptual change. This flag was always about
creating a new ABI for libstdc++ and has clearly been marked as
incompatible. Now, I've explicitly named it as such. Yay!

I'm going to let this chill a bit on mainline and then check in to
4.6.x.

-benjamin

tested x86/linux
tested x86/linux --enable-gnu-versioned-namespace