unix: port zlib definition to YAML

This _might_ tickle libz include and linking differences, as
previously we picked up the system paths. I think this may have
been a subtle bug and we should have been explicitly defining the
zlib build parameters, as we do build a custom libz.

cpython-unix/extension-modules.yml

@@ -803,4 +803,9 @@ xxlimited_35:
 xxsubtype: {}
 
 zlib:
-  setup-dist-verbatim: true
+  sources:
+    - zlibmodule.c
+  includes-deps:
+    - include
+  links:
+    - z