Blosc

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* Fix SSE2/AVX2 build issue. Fixes 352. Thanks to Thomas VINCENT
and Mark Kittisopikul.

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* Upgrade internal-complib zstd from 1.5.2 to 1.5.5.

* Zlib updated to 1.2.13.

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* Add support for SHUFFLE_AVX2_ENABLED and SHUFFLE_SSE2_ENABLED
even if AVX2 or SSE2 is not available. See PR 347. Thanks to
Thomas VINCENT.

* Upgrade internal-complib zstd from 1.5.0 to 1.5.2. Thanks to
Mark Kittisopikul.

* Many small code improvements, improved consistency and typo fixes.
Thanks to Dimitri Papadopoulos Orfanos.

* New HIDE_SYMBOLS CMake option to control the symbols exposure.
Default is ON. Thanks to Mariusz Zaborski.

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* Fix pthread flag when linking on ppc64le. See 318. Thanks to Axel Huebl.

* Updates in codecs (some bring important performance improvements):
* BloscLZ updated to 2.5.1
* Zlib updated to 1.2.11
* Zstd updated to 1.5.0

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* Updated zstd codec to 1.4.8.

* Updated lz4 codec to 1.9.3.

* New instructions on how to use the libraries in python-blosc wheels
so as to compile C-Blosc applications. See:
https://github.com/Blosc/c-blosc/blob/master/COMPILING_WITH_WHEELS.rst

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* Added `<unistd.h>` in vendored zlib 1.2.8 for compatibility with Python 3.8 in recent Mac OSX. For details, see: https://github.com/Blosc/python-blosc/issues/229.