Moving to -std=gnu11 (another case for requiring GCC >= 4.7)

[Roland McGrath]

> Any preferences on hardcoded -std=gnu1x versus a configure test to use 
> -std=gnu11 instead when available, given the prior objections to requiring 
> 4.7?

I guess I have a mild preference for using -std=gnu11 when available.
But I don't have any real rationale for that.