calibre: really fix building on master

It turns out I hardcoded the output path that qt's tarball extracts.
But that path is versioned (4.8.5 for example).
As I've already merged x-updates on my own system, my qt version was
different (4.8.4 vs 4.8.5). Made the path-guessing more flexible, so
now it should work with any 4.8.*