Use pkg-config to find apr, if --with-libapr isn't given a path.

The branch already required pkg-config, so just use it.
Remove obsolete message in configure.in, from 3.1.x release.
Rename 'configure.in' to 'configure.ac'.

The old name has been deprecated for some time.
Allow putting gmetad.py in the foreground, without changing debug level.
Add systemd unit files.
Merge pull request #75 from tomprince/systemd

Add systemd unit files.