Use the Message-Id header in patch files to check whether they have
already been applied to vlc sources.
To perform that:
- patch files are generated with the '--thread' option to generate the
  Message-Id header
- patch files are applied with 'git am --message-id ...' to keep the
  Message-Id in the commit description

Make compilation work even if we don't apply recording patches.

Signed-off-by: Romain Vimont <rom1v@videolabs.io>

We will add patches for vlc4, so we need an additional level of
directories.