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

Fixes #3, thanks to @gitisihara for providing the initial patch and
testing.

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.