From 2.10 onwards, all files are installed into versioned
directories. As everything stays compatible between revisions,
it is not technically necessary, to have revisions included in
the directories. Nevertheless, there are situations where this
is desirable. So make it available at configure time, disabled
by default.

Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Michele Tartara <mtartara@google.com>