created a SWIG interface file, and use that to generate a shared library.

factorial.cpp now contains a simple function
factorial.h, of course, contains the prototype for the function, wrapped
in an 'extern "C" {}'
factorial.i is very simple, and simply directs SWIG to scan factorial.h, and
generate a wrapper file
factorial_wrap.cpp is the generated wrapper.
libfactorial.so is created, installed, and used to link guiletest
guile can now use libfactorial by evaluating:
(load-extension "libfactorial" "scm_init_factorial_factorial_module")

FIXME: for some reason, ldconfig is not being run.