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 5671ad66)

Cabal @ ee6d1cf5

-Subproject commit e97aa58f68519db54de1c62339459ebb88aed069
+Subproject commit ee6d1cf5cefe18f6d74ed379af21d92f8b0ae92d