Optimize celt_pitch_xcorr function (for floating point)
using ARM NEON intrinsics for SoCs that have NEON VFP unit.

To enable this optimization, use --enable-intrinsics
configure option.

Compile time and runtime checks are also supported to make sure
this optimization is only enabled when the compiler supports
NEON intrinsics.

Signed-off-by: Timothy B. Terriberry <tterribe@xiph.org>