patch for configurable libffi location

[Armando Di Cianno]

Please find attached a patch to configure.ac, after making configure, 
to allow a configurable libffi location.  It is diff'd off recent cvs 
pull.

Impetus to do so:
- Many people may not have libffi, from gcc, installed, especially as 
a shared library.
- ffcall is only for 32-bit systems, so prefering libffi at the moment
- packagers will likely not put a "not compiled with gcc itself" 
version of libffi in /usr/lib, but rather into it's own directory: 
/usr/lib/libffi , /usr/include/libffi

Is this sensible?  Granted, I need to do so anyway, but it's not going 
to hurt being in configure.ac, right?  If so, can someone add this to 
CVS?

Thanks,
__Armando Di Cianno

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