The default should be what most distros and most users/devs use.
On most distros kcheckpass would be broken without PAM. Thus not
require it is a severe issue. We have had many bug reports due to
PAM missing during build and users not able to unlock.
Slackware still requires a setup without PAM, thus a cmake option
is added to not require PAM.
Do:
cmake -DPAM_REQUIRED=OFF /path/to/kscreenlocker/src
to compile kscreenlocker without PAM support.