`FFILibDir` and `FFIIncludeDir` both show up in the `rts` library's
package registration file.  We therefore must define them or else we'll
end up with spurious `@FFILibDir@` strings in the package registration.

In principle I think we could also take these as arguments to the
bindist configure but this seems simpler and I don't want to verify this
at the moment.

Test Plan: Build bindist while passing `--with-ffi-libraries=...` to
source distribution configure then try to install and use bindist.

Reviewers: austin, hvr

Subscribers: rwbarton, thomie, erikd

Differential Revision: https://phabricator.haskell.org/D3774