link errors when using extern (C) structs

[DanielG]

For the benefit of anybody who encounters a problem like this in 
the future ... originally I had my C library "header" files 
(renamed from .di to .d after the feedback from Nicholas) in a 
special 'headers/' subdir, used as an import path in dub.json. I 
simply moved those files to the source folder and everything 
started magically working.

And thanks again, Nicholas!