externals: Use upstream zlib

We don't need to depend on a custom fork for this. We can add the
library as is, and then make it excluded from the ALL target, so we only
link in the libraries that we actually make use of.

.gitmodules

@@ -47,8 +47,8 @@
     path = externals/sirit
     url = https://github.com/ReinUsesLisp/sirit
 [submodule "libzip"]
-	path = externals/libzip
-	url = https://github.com/DarkLordZach/libzip
+    path = externals/libzip
+    url = https://github.com/DarkLordZach/libzip
 [submodule "zlib"]
-	path = externals/zlib
-	url = https://github.com/DarkLordZach/zlib
+    path = externals/zlib
+    url = https://github.com/madler/zlib