In Protocol Buffers, how to import a file from the upper level directory?

I have the following code in a protocol buffer file(pcfg_lm.proto):

import "../types/language.proto";

package nlp;

message PCFGProto {
  required Language lang = 1;
}

And of course there is a proto file exists at ../types/language.proto. However, when I issue the command:

protoc pcfg_lm.proto --cpp_out=/tmp

Here is the error message:

../types/language.proto: File not found.
pcfg_lm.proto: Import "../types/language.proto" was not found or had errors.
pcfg_lm.proto:6:12: "Language" is not defined.

I think there must be some way to specify the file names in the upper level directories, without using the -I flag. But how do I do that?