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.
postBuild can be used to execute user-specific commands on the generated $out
environment before finalizing the build. For example, this hook can be used to
generate appropriate 'makeWrapper' script for binaries contained in the
environment.
The builder creates a temporary package.conf.d database in $TMP that
contains everything required to build the current package (i.e. the
transitive closure of the package's propagated build inputs). These
files are no longer installed, however, we just install the package.conf
file for the package we're actually building. This means that
package.conf.d directory in $out won't have collisions anymore, which
simplifies the with-packages-wrapper.nix a bit.
Furthermore, export "name" and "version" attributes that match those of the
underlying compiler. These changes make a ghcWithPackages-generated wrapper
look exactly like a normal GHC derivation and it can be used anywhere in
Nixpkgs where a normal GHC would be used.
