[PD] undefined symbol

[Alexandros]

I'm trying to make an external with the source being split in five 
files, where all are included in the main file that creates the object. 
This is someone else's code (given freely), not aimed at Pd, and I would 
like to maintain this structure, rather than write everything in the 
main file.

I'm using pdlibbuilder's Makefile and the compilation process succeeds 
with a few warnings about unused parameters, but when I try to create 
the object in Pd, I get the following error:

undefined symbol: set_char_to_indx

set_char_to_indx is a function defined in one of the files included in 
the main file and it is used in one place in the main file. How can it 
be solved?