XXH SIMD acceleration is not being enabled

As far as I can tell, the SIMD algorithms that have been implemented in the source code file mysys/xxhash.h are never put into use, because the preprocessor symbol XXH_ENABLE_AUTOVECTORIZE is never defined.

If I understood it correctly, this header file has been designed so that it can be included in ISA extension specific compilation units that would be compiled with additional flags, such as -mavx512 to enable those extensions.

It seems that we'd have to implement CPU detection in strings/hasher-xxh.c similar to what has been done in various files in mysys/crc32, and enable the most efficient implementation that is available at runtime.