Pass ‘make distcheck’

This lets BarnOwl build, test, and run in a build directory separate from the source tree, and fixes `make clean`, `make uninstall`, `make dist`, so that we now pass `make distcheck`.  I’m not too happy with the `cp -srf` kludge for the Perl modules (MakeMaker does not seem to have native support for a separate build directory), but I’m okay with it for now since it’s only used in the separate build directory case, and a working `make distcheck` should help prevent other build system bugs.

If we want, `make distcheck` could replace the `git archive` plus tarball wrangling steps in `scripts/do-release`.