use of dynamic libzstd.so

[eilandert]

It should be possible to use a dynamic library (libzstd.so) instead of including the static libzstd.a into the module, it's a lot easier to maintain/upgrade libzstd.so when a new version comes out instead of recompiling nginx.

I tried to create PR but didn't succeed yet.

(this had nothing to do with compiling the dynamic modules for nginx)