Which libz...?
 

I've tried to install the screensavers "kfiresaver" and "kogs" from kde-look.org and also the game "kolf" (comes with kde 3.1).
On running ./configure for these programs it always errors with "checking for libz... configure: error: not found. Check your installation and look into config.log".

What are these libz? I've looked into config.log and it doesn't help me.

that'll be the Z libs, a set of compression libraries. just install libz with "urpmi libz-devel" should sort and deps out along the way. assuming you've got urpm set up right (mandrake package manager)

ok, it got past that one (btw i'm currently urpmi'ing kdelibs because I thought that might be it). But now:

configure: WARNING: libjpeg not found. disable JPEG support.
checking for Qt... configure: error: Qt (>= Qt 3.0.3) (headers and libraries) not found. Please check your installation!
For more details about this problem, look at the end of config.log.

?

ok, so it's libjpeg you want to install, and it's devel package.

these error messages generated by configure are normally very straight forward. it's told you what's missing, so install it. simple as that. just when you install from rpm they don't installed the development side of the pacakge that is required for source installs, so there will normally be a blah-devel package to partner it.