[AD] GUI patch to allow multiple (de)selection

["Vincent Penquerc'h" <lyrian@xxxxxxxxxx>]

Hi!

This one might be the tiniest patch ever made for Allegro :)
It makes possible to deselect list items when multiple selectionning
(eg clicking on a selected item while pressing control or shift will
deselect it).

-- 
Vincent Penquerc'h
Windows NT - New Trial

--- allegro.3.9.30.original/src/guiproc.c	Sun Dec 19 12:18:48 1999
+++ allegro/src/guiproc.c	Thu Dec 23 13:45:37 1999
@@ -868,7 +868,7 @@
 		  sel[j] = TRUE;
 	    }
 	    else
-	       sel[i] = TRUE;
+	       sel[i] = !sel[i];
 	 }
       }