Hide error message if docbook not available

git-svn-id: https://nagiosplug.svn.sourceforge.net/svnroot/nagiosplug/nagiosplug/trunk@733 f882894a-f735-0410-b71e-b25c423dba1c

tools/setup

@@ -35,7 +35,7 @@ if [ `uname -s` = "SunOS" ] ; then
 		docbook=1
 	fi
 else
-	if which docbook2html ; then
+	if which docbook2html >/dev/null 2>&1; then
 		docbook=1
 	fi
 fi