Avoid bogus warning about missing zlib library

The qmake condition was missing an else statement. As a result the warning
about missing the zlib library was show for every platform but OSX.

Also we didn't pickup Qt's builtin version of zlib if available.

Change-Id: I4518aa8f3f318c315dfef762d995dd634ec440e8
Reviewed-by: Richard J. Moore <rich@kde.org>
Reviewed-by: Valerio Valerio <valerio.valerio@jollamobile.com>