syslog-ng bump and fixes, mainly for systemd support.
threaded(yes) is broken.
https://bugs.gentoo.org/show_bug.cgi?id=406623
include scl.conf and use system() source.
https://bugs.gentoo.org/show_bug.cgi?id=418317
https://bugs.gentoo.org/show_bug.cgi?id=426814
https://bugs.gentoo.org/show_bug.cgi?id=449260