Print a warning when we are given two scripts for one target. This is neither

as wide-reaching nor intensive as NetBSD's similar, but the warning uses the
same text.

Inspired by:	NetBSD

usr.bin/make/parse.c

@@ -1499,6 +1499,10 @@ ParseAddCmd(gnp, cmd)
     /* if target already supplied, ignore commands */
     if (!(gn->type & OP_HAS_COMMANDS))
 	(void)Lst_AtEnd(gn->commands, cmd);
+    else
+	Parse_Error(PARSE_WARNING,
+		    "duplicate script for target \"%s\" ignored",
+		    gn->name);
     return(0);
 }