[patch] Use __builtin_FILE and __builtin_LINE in assert implementation in C++

[Florian Weimer]

* Paul Pluzhnikov:

> Are there any strong objections to this patch which haven't been addressed?
>
> It seems that
> - it's a complete no-op in plain-C
> - it helps C++ modules
> - it has no real negatives
>
> and as such should be a "no brainer".

As I said before, use of these builtins needs to be conditionalized.

Thanks,
Florian