posix: allow standalone reuse of posix submodules.

* ext/posix/sys/msg.c, ext/posix/sys/socket.c,
ext/posix/syslog.c: Move guards to ensure even on hosts with no
sys/msg.h, we don't get an empty object file.
* local.mk (noinst_LTLIBRARIES): Add standalone compilations to
ensure the submodules really build as separate compilation units.
* NEWS: Update.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>