Build zstd on Unix

[geofft]

Python 3.14 adds a new compression.zstd module to the standard library, which depends on the zstd C library. We're currently only building this on Windows in our 3.14.0b1 releases; we need to build this on Unix too before 3.14 final.

[dtrodrigues]

Can this one be closed now due to #682 ?

[zanieb]

Yep!