Fix "make dist"

Fix "make dist"

Make various adjustments necessary to enable "make dist" to build a BIND
source tarball whose contents are complete enough to build binaries, run
unit & system tests, and generate documentation on Unix systems.

Known outstanding issues:

  - "make distcheck" does not work yet.
  - Tests do not work for out-of-tree source-tarball-based builds.
  - Source tarballs are not complete enough for building on Windows.

All of the above will be addressed in due course.