Update vendored zlib library

Author: HexDecimal

.gitmodules

@@ -2,3 +2,7 @@
 	path = libtcod
 	url = https://github.com/libtcod/libtcod
 	shallow = true
+[submodule "zlib"]
+	path = zlib
+	url = https://github.com/madler/zlib
+	shallow = true

build_libtcod.py

@@ -152,7 +152,7 @@ def walk_sources(directory: str) -> Iterator[str]:
     ".",
     "libtcod/src/vendor/",
     "libtcod/src/vendor/utf8proc",
-    "libtcod/src/vendor/zlib/",
+    "zlib/",
     *build_sdl.include_dirs,
 ]
 
@@ -169,15 +169,15 @@ def walk_sources(directory: str) -> Iterator[str]:
 sources += ["libtcod/src/vendor/stb.c"]
 sources += ["libtcod/src/vendor/lodepng.c"]
 sources += ["libtcod/src/vendor/utf8proc/utf8proc.c"]
-sources += glob.glob("libtcod/src/vendor/zlib/*.c")
+sources += glob.glob("zlib/*.c")
 
 if sys.platform == "win32":
     libraries += ["User32"]
     define_macros.append(("TCODLIB_API", ""))
     define_macros.append(("_CRT_SECURE_NO_WARNINGS", None))
 
 if sys.platform in ["win32", "darwin"]:
-    include_dirs.append("libtcod/src/zlib/")
+    include_dirs.append("zlib/")
 
 
 if sys.platform == "darwin":
@@ -199,7 +199,6 @@ def walk_sources(directory: str) -> Iterator[str]:
         "-fPIC",
         "-Wno-deprecated-declarations",
         "-Wno-discarded-qualifiers",  # Ignore discarded restrict qualifiers.
-        "-Wno-error=implicit-function-declaration",  # From zlib sources
     ],
 }