Add option to use Delphi zlib unit (instead of STATICLOAD_ZLIB)

[rlebeau]

As described in http://issuetracker.delphi-jedi.org/view.php?id=5690, IdZLibHeaders.pas should be updated to use Delphi's own System.ZLib unit. This could reduce executable size a bit (by preventing linking in multiple versions of the zlib OBJ files) and at the same time solves any 64-bit porting issues (as that seems to have been done by Embarcadero themselves already).