makeinfo 4.6 refuses positive ifset inside def*

[Akim Demaille]

The following input file works properly if the flag bar is _not
defined_.  Otherwise, it causes makeinfo to fail with:

~/src/tc % LC_ALL=C makeinfo /tmp/foo.texi                       nostromo 12:08
/tmp/foo.texi:16: Must be in address@hidden' environment to use address@hidden'.
makeinfo: Removing output file `/home/akim/src/tc/sample.info' due to errors; 
use --force to preserve.
~/src/tc % makeinfo --vers                                       nostromo Err 1
makeinfo (GNU texinfo) 4.6

Copyright © 2003 Free Software Foundation, Inc.
AUCUNE garantie n'est donnée.  Vous pouvez redistribuer des copies
de ce logiciel selon les termes de la licence GNU General Public License.
Pour plus d'informations, consultez le fichier COPYING.

----------------------------------------
\input texinfo   @c -*-texinfo-*-
@c %**start of header
@setfilename sample.info
@settitle Sample Manual 1.0
@c %**end of header

@ifnottex
@node Top
@top GNU Sample
@end ifnottex

@set bar

@deffn foo
@ifset bar
@deffnx bar
@end ifset
Documentation.
@end deffn

@bye