Getting Apple's libstdc++ debug mode into the FSF tree

[Benjamin Kosnik]

>| My suggestion: let's get Doug's changes in, and let's not tie them
>| to concept checking.  Whether or not to turn on concept checking by
>| default is an orthogonal issue.
>
>seconded.

Agreed! Let's revisit this later, get the debug library bits in, then
the debug library, and then worry about the defaults for the non-debug
library. 

I also remember some compilation time issues, but that might be solved
with PCH, who knows. In any case, a serious review of this functionality
is needed.

-benjamin