multilanguage compilation

Hello,

Coming back to an old piece of software, I found the following problem 
when trying to build it under GPS :

- this software is mainly made of Ada files, with a single C module 
(i.e. 1 .c + 1 .h files)
- as far as I can remember, I had nothing to do - except to select Ada 
and C languages in the project wizard - to get the C functions compiled, 
put in a .a library, and linked with the Ada application.
- now I can't get the C file compiled, neither linked with Ada modules - 
  gnatlink says "undefined reference to `C_name_of_interfaced_function'"

I lost the former .gpr file, so I can't compare, and I couldn't find the 
solution in GPS documentation. What did I do wrong?

Thanks for help

André Laurens