[flac-dev] Improve encoding speed on older Intel CPUs

[lvqcl]

The commit http://git.xiph.org/?p=flac.git;a=commit;h=e9d805dd4374cead02a4e991515bae3bf9547e43
changes the code of functons that calculate autocorrelation.
However, the new code works 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 in realtime.
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