errno.3: Recast the advice against manually declaring 'errno'

Recast the advice against manually declaring 'errno' to
a more modern perspective. It's 13 years since the original
text was added, and even then it was describing old behavior.
Cast the description to be about behavior further away in
time, and note more clearly that manual declaration will
cause problems with modern C libraries.

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man3/errno.3

@@ -614,18 +614,18 @@ if (somecall() == \-1) {
 .EE
 .in
 .PP
-It was common in traditional C to declare
+On some ancient systems,
+.I <errno.h>
+was not present or did not declare
+.IR errno ,
+so that it was necessary to declare
 .I errno
 manually
 (i.e.,
-.IR "extern int errno" )
-instead of including
-.IR <errno.h> .
+.IR "extern int errno" ).
 .BR "Do not do this" .
-It will not work with modern versions of the C library.
-However, on (very) old UNIX systems, there may be no
-.I <errno.h>
-and the declaration is needed.
+It long ago ceased to be necessary,
+and it will cause problems with modern versions of the C library.
 .SH SEE ALSO
 .BR errno (1),  \" In the moreutils package
 .BR err (3),