Cleanup.

CMake script cleanup, no functional changes.
Fix build on Ubuntu 11.10 and Qt 5.0.

For some reason Vibe needs to be linked to zlib explicitely, at least
on Ubuntu 11.10 using Qt 5.0 downloaded from qt-project.org.

This doesn't happen on ArchLinux with our Qt 5.x git packages, probably
because QtCore is already linked to zlib.