Use pkg_config for zstd and make it private (cherry-pick !4507)
This is necessary to avoid the cmake error that zstd (config) is not found in Lbcom and downstream projects.
Could not find a package configuration file provided by "zstd"...
Existing releases will unfortunately still exhibit this problem.
(Sometimes one does not see this problem, but then the zstd cmake is wrongly found in the LbEnv installation rather than from the system.)