Fixes to enable building with -mavx, for Sandybridge CPUs. (#992)
Fixes to enable building with -mavx, for Sandybridge CPUs. (#992)

The -mf16c (i.e. packed-half conversion instructions) weren't added until the
Ivy Bridge CPU generation (https://en.wikipedia.org/wiki/F16C#CPUs_with_F16C).
Therefore, code dependent on those intrinsics must be conditioned on __F16C__ -
not __AVX__.
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