Require PAM by default and provide an option to not require it

Summary:
The default should be what most distros and most users/devs use.
On most distros kcheckpass would be broken without PAM. Thus to 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.

CCMAIL: distributions@kde.org
CCMAIL: kde-distro-packagers@kde.org

Test Plan: Tested cmake with and without that option

Reviewers: #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D4768