Update IdZLibHeaders to use System.Zlib unit

```
The System.Zlib unit has been available in Delphi/C++Builder since at least RAD 
Studio 2006.  The IdZLibHeaders unit should be updated with a new {$DEFINE} 
that allows Indy to use that unit instead of using its own local copy of ZLib.
```

Original issue reported on code.google.com by `gambit47` on 13 Jul 2015 at 5:50
