Dependencies for libffi-0.2.1

When GHC is configured without --with-system-libffi, it will bundle its own copies of libffi, one of them statically linked and the other dynamically linked. This flag will force linking against the static copy of libffi that GHC bundles. This avoids a GHC bug (https:/gitlab.haskell.orgghcghc-issues15397) that can arise when the linker confuses the system's dynamic libffi with GHC's own dynamic libffi. Note that this flag only works when GHC is configured without the --with-system-libffi option. This is the case for most GHC binary distributions, such as those provided by ghcup. If you are using a GHC that was configured with --with-system-libffi, however, you will need to disable this option and link against the system's version of libffi instead.