simd.h include file logic cleanup.
simd.h include file logic cleanup.

immintrin.h seems to always include a bunch of other files that we
included explicitly, so those extra includes can go away.

Also, the guard around x86intrin.h needs to ensure that gcc version is
at least 4.4 and it's also unnecessary unless f16c is requested.

lgritz/oiio,YangYangTL/oiio,lgritz/oiio,lgritz/oiio,YangYangTL/oiio,micler/oiio,OpenImageIO/oiio,bdeluca/oiio,YangYangTL/oiio,bdeluca/oiio,OpenImageIO/oiio,micler/oiio,micler/oiio,bdeluca/oiio,OpenImageIO/oiio,lgritz/oiio,micler/oiio,bdeluca/oiio,OpenImageIO/oiio,YangYangTL/oiio