Log Message:
pkgin: Update to 20.5.1.

This is quite a disruptive change, but has been tested in various ways across
Darwin, Minix, and SmartOS, so I'm reasonably confident I caught most issues.

With the switch to automake the pkgsrc Makefile has been able to be simplified
a lot, and I also took the opportunity to remove the mostly useless MESSAGE
file.

## Version 20.5.1 (2020-05-25)

 * Improve make portability.

## Version 20.5.0 (2020-05-25)

 * Switch to automake and overhaul mandatory configure options.
 * Remove unused code and pointless configure tests, cleaning up includes
   and headers where appropriate.
 * New version number scheme (yy.m.x).  The previous scheme had no relevance,
   this one at least gives users some clue about how up-to-date their version
   is.