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

[Gabriel Dos Reis]

Benjamin Kosnik <bkoz@redhat.com> writes:

| >I'm in favor of concept checking being on by default, period.
| 
| You and me both. However, it does impact codegen, or it did the last
| time somebody looked closely. I thought that was the reason it was off
| by default. (As well as instantiation difficulties on non-ELF platforms,
| but perhaps this has been resolved now.)

We also got some reports about false positives.

-- Gaby