mod_systemd: if SELinux is available and enabled, log the SELinux

mod_systemd: if SELinux is available and enabled, log the SELinux
context at startup, since this may vary when httpd is started via
systemd vs being started directly.

* modules/arch/unix/mod_systemd.c (systemd_post_config):
  Do nothing for the pre-config iteration.
  Log the SELinux context if available.

* modules/arch/unix/config5.m4: Detect libselinux.

Have at least one CI job build mod_systemd.

Github: closes #422

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1916344 13f79535-47bb-0310-9956-ffa450edef68