Speed up build and avoid xmlto issues

This solves #190 using another approach: touching all files after they have been stripped using a reference time, so that all files get exactly the same modification time and don't trigger rebuilds. As the zbeacon patch issue is already fixed, the only effect this should have now is to speed up building from a clean/fresh tree.

This patch also causes zmq, czmq, fmq and zyre builds to skip generating documentation files, which also speeds up compilation and avoids the build issue with xmlto/asciidoc cited [here](https://github.com/koreader/koreader-base/pull/190#issuecomment-47452031).