Zstandard support for setup

[Achim Gratz]

Jon Turney writes:
> This should check for ztd in configure.ac using PKG_CHECK_MODULES,
> rather than just assuming -lzstd is going to work.

Done.

> + using namespace std;
>
> Please don't

Done.

https://repo.or.cz/cygwin-setup/local.git/patch/fff4b1448072efe5ebb6549cfe04bc15344a3973

> compress_zstd.h:
>
> +/* this is the parent class for all compress IO operations.
> + */
>
> Comment is incorrect

Done in the above, additionally remove the same copy&paste comment from
two existing files:

https://repo.or.cz/cygwin-setup/local.git/patch/abf5afbfce47d1942f6d4c2f62fc8b1566920e2e


> I'd suggest keeping potentially useful ones under #ifdef DEBUG/#endif
> rather than just removing them all.

Later...


Regards,
Achim.
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