Pgxs - How to reference another extension

Hi,

I am trying to create an extension that delegates some calls to btree_gist functions:

DirectFunctionCall5Coll(
                gbt_text_consistent, …other arguments);

Basic PGXS Makefile does not work - I get linker error:

Undefined symbols for architecture arm64:
  "_gbt_text_consistent", referenced from:


Anyone could provide me with some hints?

—
Michal