Revert "rts: Drop redundant flags for libffi"

Authored by bgamari on Dec 10 2019, 3:07 PM.

Description

Revert "rts: Drop redundant flags for libffi"

This seems to have regressed builds using --with-system-libffi
(Trac #17520).

This reverts commit 3ce18700f80a12c48a029b49c6201ad2410071bb.

Details