Update to latest Cabal 1.18 branch tip (again)

This was already performed in 37d6e2c5 but accidentally
reverted as a side-effect of 9976c2ec.

So this update pulls in the fix into ghc-7.8 for #8266 again.

Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>

Cabal @ d310d87c

-Subproject commit e97aa58f68519db54de1c62339459ebb88aed069
+Subproject commit d310d87c2c445f52987169a2ce4da03c14070918