Handle differences in PAM headers

Summary:
FreeBSD has neither pam_ext.h nor pam_syslog(), so check for headers
and add defines as appropriate.

Test Plan: Builds on FreeBSD, now. Needs a test-build on Linux.

Reviewers: jriddell, #plasma

Reviewed By: jriddell, #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D6000