distrib/configure: Carry FFI include/lib paths from source distribution

`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

distrib/configure.ac.in

@@ -12,8 +12,15 @@ dnl--------------------------------------------------------------------
 
 FP_GMP
 
+dnl Various things from the source distribution configure
 bootstrap_target=@TargetPlatform@
 
+FFIIncludeDir=@FFIIncludeDir@
+FFILibDir=@FFILibDir@
+AC_SUBST(FFILibDir)
+AC_SUBST(FFIIncludeDir)
+
+
 # We have to run these unconditionally as FPTOOLS_SET_PLATFORM_VARS wants the
 # values it computes.
 AC_CANONICAL_BUILD