This commit fixes several issues with the systemd unit installation:
1) The path for the units varies by distro.  For example, on RedHat,
   it is /usr/lib/systemd/system, but on Debian it is
   /lib/systemd/system.  This change uses pkg-config to lookup the
   correct path.
2) The systemd units are now installed by default, if the system has
   systemd installed, as determined by pkg-config.  If the user has
   systemd installed, but is not using it, the two text files installed
   are moot and harmless.  This eliminates the systemdenable and
   systemddisable waf commands.  If you are using systemd and wish to
   disable the units for some reason, use the built-in mechanism of
   `systemctl disable ntpd.service ntp-wait.service`.
3) The paths for the executables (ntpd and ntp-wait) inside the unit
   files are now set using subst so they match the --sbindir and
   --bindir, respectively, as set at waf configure time.