[v3] multi-pch

[Benjamin Kosnik]

> > From this, it seems as if moving to std + TR1 as the PCH of choice
> > provides real benefits, but precompiling every interface (ie std + TR1
> > + ext) does not.
> 
> That conclusion applies if you're interested in making the testsuite
> run fast.  

Err. Yes. I want that.

Thus the timings for running the testsuite...

>However, if you have a large program that uses ext but not TR1,
> extc++ will win, and if you have a large program uses neither TR1 nor
> ext, stdc++ will win.

Sure. In general, the less you include, the better off you are.

In any case, I think more options in PCH-land can only help us.

-benjamin