versioning C++ includes

[Benjamin Kosnik]

> Completely agreed.

[snip]

> Again, completely agreed.  For some time I had a patch in my local
> tree trying to implement that, but while it was easy for me to
> convince Cpp to search in the right dir, our configury machinary was a
> bit relunctant ... maybe because finally I'm not that familiar with it

Great. I was hoping you would weigh in on this!

I remember the configuration for this kind of stuff being kind of
opaque. Hopefully this will be a good excuse to make it easier for
everybody to understand.

Patch pending...

-benjamin