[Patch] libstdc++/29989

[Benjamin Kosnik]

> If the #defs are removed, I thought it might be wise for at least one
> version to perhaps replace them with something like this, as the error
> message you get from something being defined as a little difficult to
> understand...
> 
> 
> #ifdef min
> #error min should not be defined as a macro
> #endif

This is a great idea Chris. I also think this would lead to the most
useful error messages.

-benjamin