Replace ZLib With A Submodules (v3.0)

[TheBrokenRail]

This PR does two things:

1. It modifies the CMake build to use the system's ZLib, rather than the vendored version.
2. It replaces the vendored version with a submodule of the same version.

One thing to note: the submodule version is ZLib is almost identical to the old version, except for one change:

--- zlib/gzguts.h
+++ zlib-old/gzguts.h
@@ -37,13 +37,15 @@
 
 #if defined(__TURBOC__) || defined(_MSC_VER) || defined(_WIN32)
 #  include <io.h>
+#else
+#  include <unistd.h>
 #endif
 
 #if defined(_WIN32)
 #  define WIDECHAR
 #endif
 
-#ifdef WINAPI_FAMILY
+#if defined(WINAPI_FAMILY) || defined(_WIN32)
 #  define open _open
 #  define read _read
 #  define write _write

I do not know why the old version of ZLib had undocumented changes, but the submodule version doesn't.

[iProgramMC]

So how did you embed those changes into the submodule zlib?

What I know is that I had to add those because the game wouldn't build otherwise.

[TheBrokenRail]

You can't easily patch submodules. I'd suggest just adding the WINAPI_FAMILY define to the VS build so the patch isn't needed.

[TheBrokenRail]

I'm going to mark this as a draft for now