Provide ZLIB::ZLIB as a CMake alias when shared libraries are disabled

[ccawley2011]

This matches the behaviour of FindZLIB from standard CMake installations.

[Vollstrecker]

Point 1: Why do you checkif the target exists, if you could check for the option if the target should be created?

Point 2: FindZLIB tries to provide something, but the point in the named targets is to know what you link against. If you you want a static linking, there's a target. Imagine you expect dynamic linking and want to install the dll in your script, but there is none, you would have to add additional checks. Yes, with your changes in the config you mimic the old behaviour, but if someone includes the sources Idmeand them to check if there are changes. Such situations happen with apis all the time, so it's not asked too much to check if all the targets are still correct (and if you link against ZLIB:::ZLIB and don't set any options you'll have no problems in this case), and instead of checking for existing dll's I consider it better to check for the version and just use the right targets in you build instead of hoping to get sommething.