Ignore --with-foo=external arguments in subdirs

Signed-off-by: Ralph Castain <rhc@open-mpi.org>

config/opal_config_subdir_args.m4

@@ -62,6 +62,8 @@ do
 	    ;;
         --with*=internal)
             ;;
+        --with*=external)
+            ;;
 	*)
 	    case $subdir_arg in
 	    *\'*) subdir_arg=`echo "$subdir_arg" | sed "s/'/'\\\\\\\\''/g"` ;;