_GLIBCPP_FULLY_COMPLIANT_HEADERS: Why not turned on?

[Benjamin Kosnik]

cool. This was done just to speed up day-to-day compiles. I'm not quite 
sure it's still worth it, and should probably be removed.

If you could though, please try to see how significant the 
slowdown/speedup is when this is turned on/off. I'd appreciate some kind 
of analysis.

there's no doubt that we should really turn this on this for conformance 
reasons (it's on when I do conformance testing), so I'm inclined to do it 
anyway. I am curious about performace though.

-benjamin