How to build without libzstd

[sulincix]

I design minimal distribution. It does not have zstd .

d080b17

this commit add zstd dependency. I think we should add disable options.

[ximion]

Is zstd really that large? Especially since it is depended on by so much stuff nowadays!
Since it's quite a bit faster, I expect zstd to actually become a non-disableable default, but for now we can temporarily make it optional.