(EILSEQ): Include <time.h> and <wchar.h> if <errno.h> does not define

EILSEQ, because BSD/OS 4.1 defines EILSEQ in <wchar.h>, which requires
<time.h>.  Define a replacement EILSEQ to be ENOENT, not EINVAL, since
callers may want to distinguish EINVAL and EILSEQ.

lib/unicodeio.c

@@ -37,8 +37,25 @@
 #ifndef errno
 extern int errno;
 #endif
+
+/* Define EILSEQ and ENOTSUP as portably as possible.  Some
+   nonstandard systems, like SunOS 4, don't have EILSEQ.  Others, like
+   BSD/OS 4.1, define it in <wchar.h>.  Callers that use EILSEQ and/or
+   ENOTSUP and that want to be portable to these nonstandard systems
+   should mimic the following includes and defines.  */
+
+/* BSD/OS 4.1 wchar.h defines EILSEQ, but it requires FILE (defined in
+   <stdio.h>, included above) and struct tm (defined in <time.h>) to
+   be declared.  */
+#if HAVE_WCHAR_H && ! defined EILSEQ
+# include <time.h>
+# include <wchar.h>
+#endif
+
+/* Do not define EILSEQ to be EINVAL, since callers may want to
+   distinguish EINVAL and EILSEQ.  */
 #ifndef EILSEQ
-# define EILSEQ EINVAL
+# define EILSEQ ENOENT
 #endif
 #ifndef ENOTSUP
 # define ENOTSUP EINVAL