[cmake-developers] FindZLIB module should find debug and, release variants

[Michael Scott]

> Please add each of the _DEBUG and _RELEASE configurations if and
> only if the corresponding variables are set.  Grep for mention of
> IMPORTED_CONFIGURATIONS in other Modules/Find*.cmake files for
> examples.

I'm assuming that there should be a IMPORTED_LOCATION property defined 
in all cases as well? The other Find modules aren't entirely consistent 
for this point, but it would make sense to me to set it to ZLIB_LIBRARY, 
similar to how FindOpenSSL behaves.

Cheers,
Michael