This partially reverts commit a7577667d6.
The musl C library does not use an internal cdefs.h header. As a result,
the definitions in sys/cdefs.h have no effect. This breads the build as
follows:
In file included from adjfreq_linux.c:23:0:
../src/ntpd.h:448:8: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ before ‘void’
__dead void fatal(const char *, ...)
^~~~
../src/ntpd.h:450:8: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ before ‘void’
__dead void fatalx(const char *, ...)
^~~~
Refer the related musl FAQ about cdefs.h:
http://wiki.musl-libc.org/wiki/FAQ#Q:_I.27m_trying_to_compile_something_against_musl_and_I_get_error_messages_about_sys.2Fcdefs.h
Keep the compiler attribute definition.
Remove also from noinst_HEADERS.

Solaris and illumos based distributions) which don't have it.

e0ed8ed664 went too far and broke the
backward compat headers for Linux systems.

This also removes the need for __weak_alias shims

do not have it, and a getifaddrs substitute for Solaris 10.

There are crypto hash function symbol overlaps between libmd and libcrypto on
FreeBSD, which causes hashing of the certificates store to fail as a
side-effect when using TLS constraints. Since all we really need is MD5, just
use the embedded OpenBSD versions instead.

Due to some circular dependencies on Solaris, this needs to be defined
as a macro. We should just remove SA_LEN usage here next.

If sa_family is not set, sa_len should be zero.

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