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.