PATCH (libstdc++-v3): Fix libstdc++/7680 (and a reference fix for the dynamic visibility of C99 features in system headers)

[Loren James Rittle]

>I'm wondering how much typing you would save with CPP...

I wondered the same thing.  How do I put a #if region inside a macro
defined with #define?

I need to define something like this to tighten it:

#define _GLIBCXX_C99_BIND(a,b) \
#if (a) \
  using ::b; \
#endif \