release: don't include temp files

release: don't include temp files

When we prepare the release, we generate the manual in various formats,
so that it can be consulted locally without needing the miriads of tools
needed to generate it.

However, this creates the temporary .br2-external.* files in the output
directory, and those end up in the release tarball.

This is not a problem in practice, but is not clean.

Run 'distclean' in the output directory, to get rid of everything but
the generated documentation.

Reported-by: Danomi Manchego <danomimanchego123@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>