src: fix without-intl build

In 5d38d543, an additional property in node_config.cc was added
whose definition depends on having the local `env` variable declared,
which in turn depended on `NODE_HAVE_I18N_SUPPORT` being defined.

Moving `env = ...` out of the `#ifdef` block allows building via
`./configure --without-intl` again.

PR-URL: https://github.com/nodejs/node/pull/6820


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>