zstd system library suppport

[etbe]

Please make the CMakeLists.txt file check for /usr/include/zstd.h and /usr/lib/x86_64-linux-gnu/libzstd.a and use them instead of building an inline copy of zstd if available.

[richgel999]

If we do this we're now introducing a dependency, making the library more brittle.

[etbe]

I think that having multiple copies of zstd being used on the same system makes it more brittle. Then if there's an issue with zstd it has to be solved in multiple places. If there's an issue that's not obvious (EG occasional data corruption) then things will fail in unexpected ways.

Also I'm not suggesting a hard dependency, just a cmake check for it and use the inline code if it's not installed.

[richgel999]

We're going to be working on our cmake situation more and can add this as an option.