Improve encoding speed on older Intel CPUs.
The commit http://git.xiph.org/?p=flac.git;a=commit;h=e9d805dd4374 changed the that calculate autocorrelation. However, the new code worked slightly (about 4%) slower on Core 2, but with the new presets the speed decrease can reach ~25%. This patch enables both old and new functions and chooses between them at runtime. Patch-from: lvqcl <email@example.com>