Fix compilation warnings in ZLib (MSVC)
Yes, we are changing the Zlib code. This is dangerous and uncool.
Fortunately, these are just some implicit casts.
Add ZLib as a built-in dependency
I don't know if this is good or bad. This lets libgit2 compile cleanly
on any platforms without any external dependencies, but adds a little
bit of bloat...
Let's test this out and see what happens.