haskell-gi, gi-gdkx11: fix builds

The `haskell-gi` build fails its doctests because of a missing
library; I'm not 100% convinced that setting it to `dontCheck` is the
right thing to do, but I don't have a better idea at the moment.

The `gi-gdkx11` build fails because, surprise, Gdk-X11 isn't found; by
looking around in my store, I found that that that library seems to
live in gtk3 these days; this override is just a stop-gap, though,
I've also submitted the change to cabal2nix that I believe will fix
the automatic generation of the package in the future.