sys.msg: only enable SysV messaging where it is supported.

* configure.ac (AC_CHECK_HEADER): Add sys/msg.h.
(AC_CHECK_FUNCS): Add msgrcv, msgsend.
(HAVE_SYSV_MESSAGING): Define if the above are all present.
* ext/posix/sys/msg.c (LPOSIX_2001_COMPLIANT): Change from using
this...
(HAVE_SYSV_MESSAGING): ...to this.

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