You'll probably want to add a C header file, but as of the current version preprocessor directives do not work... so what do? On systems with GCC run the file through this command:
cpp -P /usr/include/unprocessedheader.h -o myprettyheader.h
Note that because preprocessing is not suppored in FFI, C/C++ macros are not supported either. You'll probably still have to write a small wrapper in C unless your library has an exceedingly simple public API.