RFC: Top level configure: Require a minimum version 6.8 texinfo

[YunQiang Su]

> I think that is too new.
> We still allow building gcc e.g. with GCC 4.8 from ~ 10 years ago and
> I think various boxes where people regularly build gcc will have similarly
> old other tools.
> So, bumping requirement from ~ 20 years old tools to ~ 10 years old tools
> might be ok, but requiring ones at most 2 years old will be a nightmare,

I agree.
Lots of toolchains distributed by CPU vendors are supposed to support
CentOS 6, or at least 7.