Update to latest Cabal 1.18 branch tip

This update pulls in the fix for #8266

(recommended add-on reading for those interested in OSX linker peculiarities:
https://github.com/haskell/cabal/issues/1660#issuecomment-33701508 )

Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>
(cherry picked from commit 5671ad66b8c938939a44c883002caa4e13be098c)