maint: support _POSIX_C_SOURCE in lieu of _POSIX_VERSION.

On OpenBSD 5.6, _POSIX_VERSION is set to just 199009L by default,
and messing with _XOPEN_SOURCE or setting _POSIX_VERSION directly
changes the C library header declarations.  It seems that
_POSIX_C_SOURCE is defined to 200112L by default, indicating that
the newer functions luaposix wants to bind to really are available.
* configure.ac (POSIX_EXTRA_CPPFLAGS): Elide _XOPEN_SOURCE
definition on OpenBSD.
* ext/include/_helpers.c (LPOSIX_2001_COMPLIANT): New preprocessor
symbol, defined when either _POSIX_VERSION or _POSIX_C_SOURCE
are bigger than 200112L.
* ext/posix/stdlib.c, ext/posix/sys/msg.c,
ext/posix/sys/socket.c, ext/posix/sys/times.c,
ext/posix/syslog.c, ext/posix/unistd.c: Predicate on new
LPOSIX_2001_COMPLIANT definition instead of _POSIX_VERSION.

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