This is a follow up on the quick fix for exporting from the php-src
Github mirror[1]. Instead of hard-coding everything in the code, we
introduce a configurable `gh_url`, and use it if given. We cannot use
the repository URL, though, because the toplevel folder inside the
downloaded archive would have a different name (the shortened revision
number is expanded again); downloading from the codeload subdomain
gives the desired result.
[1] <9e1ca1002a>
