[PATCH] Enable AVX2 optimized memset only if -mavx2 works

[H.J. Lu]

On Thu, Jul 10, 2014 at 11:24 AM, Roland McGrath <roland@hack.frob.com> wrote:
> Changes that claim to be performance improvements always must be
> accompanied by reports of benchmark data.

This patch disables AVX2 optimization if AVX2 isn't supported.  Otherwise,
glibc fails to build:

https://sourceware.org/ml/libc-alpha/2014-07/msg00007.html

-- 
H.J.