Fix Gap.extensionToString

`Gap.extensionToString` is broken in GHC 7.6.*. It converts to invalid options like "-XEnableExtension OverloadedStrings".

There is an API that converts `Extension` to `String` properly.
http://hackage.haskell.org/packages/archive/Cabal/1.16.0.3/doc/html/Distribution-Text.html