make @iftex case work in texinfo-5.1

doc/whatis.texi

@@ -35,10 +35,11 @@ services can authenticate each other.
 @end macro
 @end ifinfo
 
-@tex
+@iftex
-@def@xsub#1{$_{#1}$}
+@macro sub{arg}
-@global@let@sub=@xsub
+@textsubscript{\arg\}
-@end tex
+@end macro
+@end iftex
 
 @ifhtml
 @macro sub{arg}