errno: port to LynxOS 178 2.2.2

errno: port to LynxOS 178 2.2.2

Problem reported by Joel Brobecker in
<http://lists.gnu.org/archive/html/bug-gnulib/2012-10/msg00088.html>.
* doc/posix-headers/errno.texi (errno.h): Document this.
* lib/errno.in.h (EILSEQ, GNULIB_defined_EILSEQ) [!EILSEQ]: New macros.
* lib/strerror-override.c, lib/strerror-override.h (strerror_override):
Supply a string for EILSEQ.
* m4/errno_h.m4 (gl_HEADER_ERRNO_H): Check for EILSEQ.