[PATCH] Silence clang -Wignored-attributes when building with clang

[Florian Weimer]

* Matt Turner:

> Building glibc with clang trips -Wignored-attributes on the alias
> redirections glibc uses intentionally, e.g. the long_double_symbol
> machinery where a public name aliases a weak alias of an internal
> symbol.
>
> Detect whether CC is clang (cc-clang) and add -Wno-ignored-attributes
> to the warning flags.

Could you add something to the commit message that explains why these
warnings are harmless, please?

Thanks,
Florian