--enable-ffi in the user's guide should be --enable-libffi

[Gwen Weinholt]

From doc/ikarus-scheme-users-guide.tex:
"To enable
\texttt{libffi} while building Ikarus, one must pass
\texttt{--enable-ffi} to the \texttt{configure} script along with
the paths to \texttt{libffi}'s include and lib directory."

But the configure script says something else:
  --enable-libffi enable support for libffi.

[Abdulaziz Ghuloum (aghuloum)]

Fixed in 1679. Thanks.