Consider adding an environment variable for dynamic linking

[happysalada]

Would you be ok with an environment variable that could be used to specify dynamic linking ? At the moment dynamic linking relies on the "pkg-config" feature. having the condition include also an environment variable would

• make it easy to use dynamic linking in a dev environment (even for dependencies of dependencies). It could save precious compile time
• Make it easy for some linux distributions to avoid static compilation and rely on dynamic linking (where static compilation fails for some reason, you could rely on a pre-built version, (one example of this is nixos).

if you are ok with this, I would be happy to make a PR. how about the name CARGO_ZSTD_SYS_DYNAMIC_LINKING for the environment variable ?

Last but not least, thanks a lot for the great library!

[Artturin]

another dev from nixpkgs checking in :)

i came to find if there was a way to not need to patch nushell to make it's dependency use pkg-config.