Mention Tru64 5.1 fgrep limitation with emtpy patterns.

[Ralf Wildenhues]

OK?  This is
<http://thread.gmane.org/gmane.comp.gnu.libtool.bugs/7369/focus=9821>.
grep and egrep work fine with empty patterns.

Thanks,
Ralf

    Mention Tru64 5.1 fgrep limitation with emtpy patterns.
    
    * doc/autoconf.texi (Limitations of Usual Tools): Update.

diff --git a/doc/autoconf.texi b/doc/autoconf.texi
index 8d2e952..c2772fe 100644
--- a/doc/autoconf.texi
+++ b/doc/autoconf.texi
@@ -17809,6 +17809,8 @@ Limitations of Usual Tools
 not work on long input lines.  To work around these problems, invoke
 @code{AC_PROG_FGREP} and then use @code{$FGREP}.
 
+Tru64/OSF 5.1 @command{fgrep} does not match an empty pattern.
+
 
 @item @command{find}
 @c -----------------