Support for building with zlib-ng

zlib-ng provides substantially more performance than zlib, for both compression and decompression.

While zlib-ng has a "zlib compat" mode, where it exposes exactly the same symbols as zlib, that's not always possible to use, because that would potentially conflict with libraries that link with system zlib.

Would you be interested in a patch that adds optional support for building libgit2 with zlib-ng?