Provide zlibWrapper as a shared library

**Is your feature request related to a problem? Please describe.**
Currently, making use of the zlibWrapper requires a project copying a few (5) source files from this project, and building those files as part of their build. It would be simpler to simply link to a `zlib-wrapper.so` library provided by zstd.

**Describe the solution you'd like**
The build system of zstd would be extended with a target to build zlib-wrapper as a (shared) library and register it so it gets installed as part of the regular install phase.

**Describe alternatives you've considered**
None.

**Additional context**
This came up while proposing to contribute zstd-decompression support (via zlibWrapper) to mandoc (see: https://inbox.vuxu.org/mandoc-discuss/Z5i0H+XrKVrZqAXB@asta-kit.de/).