Language pragmas needed by GHC HEAD (v7.9)

These pragmas are necessary to compile with bleeding edge GHC. Similar changes were needed to many libs, in order to get e.g. `arithmoi` etc. compilable.

_Note:_ unfortunately `src/Codec/Picture/Png.hs` does not build yet, due to (allegedly) illegal use of runST. Not sure whether this is a Juicy or GHC bug.