debuild fixes

This fixes up the debuild build system so that a Launchpad build will create a binary which prints the correct revision number with purr-data -version (i.e., something like 20180210-rev.3b326993 rather than -rev. which happens when packages/Makefile tries to regenerate s_stuff.h outside of a working copy of the git repo). It does this by adding a pre-generated s_stuff.h to the source tarball and making sure that this doesn't get overwritten during a build from the tarball.

This only touches the debuild stuff, so it doesn't affect the rest of the build system in any way. Tested on Launchpad, local builds using debuild also still work as before.