When building `reflex-dom` for GHCJS, the following dependencies are not
needed and will fail to build: glib, gtk3, webkitgtk3,
webkitgtk3-javascriptcore. Now we explicitely need the dependencies
needed for building for GHCJS.
This also:
1 Builds Setup.hs with ghcjs, which (among other things) defines
__GHCJS__ and ghcjs_HOST_OS during pre-processing.
2 Fixes ghc-paths to point at ghcjs and use NIX_GHCJS_* env-vars.
3 Boots ghcjs into $prefix/lib/$compiler.
