fix from-scratch installation

openpkg/openpkg.spec

@@ -742,7 +742,7 @@ Provides:     OpenPKG
     [ ".$ngrp" = . ] && ngrp="%{?l_ngrp}%{!?l_ngrp:%{l_fsgrp}-n}"
 
     #   determine prefix
-    prefix="%{l_prefix}"
+    [ ".$prefix" = . ] && prefix="%{l_prefix}"
     prefix=`echo "$prefix" | sed -e 's;//*;/;g' -e 's;/$;;'`
 
     #   determine user/group ids