dev-libs/libffi: allow includedir= override, bug #643582

dev-libs/libffi: allow includedir= override, bug #643582

It's not a fix for bug #643582 as includes location did
not change. But a step towards it.

Changes are:
- updated to EAPI=6
- backported upstream commit
    https://github.com/libffi/libffi/commit/982b89c01aca99c7bc229914fc1521f96930919b
  ("Install public headers in the standard path")
- but overrode default location as
    --includedir=/usr/$(get_libdir)/${P}/include/
  to keep python from breaking: bug #643582#c2.

Reported-by: William L. Thomson Jr.
Bug: https://bugs.gentoo.org/643582
Package-Manager: Portage-2.3.19, Repoman-2.3.6