Another attempt at fixing the build hook

It turns out we can't `cp` over an existing file. However, we can `mv` it
or `rm` it. If we do that, `/proc/self/exe` will be pointing to the
wrong place or gets ` (deleted)` put into the symlink target.

Therefore, we need to `readlink()` before it changes to know what to
exec.
Merge pull request #6 from scraperwiki/exec-fix

Another attempt at fixing the build hook