configure: check for stdatomic.h

Since this is a C11 feature, it requires -std=c11.

Not actually used for anything yet, that will be added in the following
commits.

This merges libav commit 13f5d2bf75b95a0bfdb9940a5e359a719e242bed.

Signed-off-by: Wan-Teh Chang <wtc@google.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
