Do not distribute generated files in the release tarball

Generated files should always be built from source and never be present in
VCS repositories and only autotools generated files should be in tarballs.
This ensures that they get built as often as possible and bugs with that
process are discovered early.

Signed-off-by: Paul Wise <pabs3@bonedaddy.net>
Signed-off-by: Simon Horman <horms@verge.net.au>