build: Improve PAM prefix handling or remove it

Currently the PAM prefix can be set at configure time via --with-pam-prefix, otherwise it is set to $sysconfdir by default. Several problems arise:

• If no install prefix is explicitly provided via --prefix and --sysconfdir is not used either, PAM_PREFIX is set to NONE because it is set too soon in configure.ac. (fixed by !52 (merged))
• When properly set to a value other than /etc, it seems not to be usable at least on linux systems, where only /etc is a valid choice.
• Incidentally, checks for the presence of PAM configuration files are hard-coded without regard to this prefix.

If the ability to choose this prefix serves any purpose, it should be improved, otherwise it should just be removed.

Because if this prefix is not correctly set, /etc/pam.d/xfce4-screensaver is not correctly installed and locking fails, forcing to kill xfce4-screensaver from a tty to be able to access your session again (the build warning about this can be missed, and anyway it was always displayed before c1ef19a7).