Patch: fix warning

[Anthony Green]

Torsten Rupp <torsten.rupp@aicas.com> writes:

> Dear developers,
>
> the file debug.c may produce a warning of missing prototypes when not 
> compiled with FFI_DEBUG defined. The attached patch wrap the 
> functions in debug.c with a #ifdef FFI_DEBUG to fix this warning.

I just modified Makefile.am to not build debug.c unless you've
configured with --enable-debug.  Technically this alters the ABI, so
I'll have to tweak the libtool version.

AG