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.
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.