This is used to ensure that the time is written to the RTC when it is
synced. Problem noticed and patched by Christian Weisgerber.

Layout build machinery closer to LibreSSL to make code sharing easier.
Split the big Makefile.am into src/compat.

Thanks to @chneukirchen for the suggestion.

This makes it easier to see what is supported by each OS, and to improve
them individually.

Well, support may be too concrete of a term. There is a lot that is
missing in OS X, and not everything can be implemented with portability
shims. The time APIs in OS X seem to have frozen with NeXTSTEP and never
advanced with POSIX.

First attempt at building a new OpenNTPD portable project, based on the
latest OpenBSD source.