seafile: build against libevhtp 1.1 instead of 1.2
This fixes multiple issues like upload/download problems on seahub
and also makes it possible to use the new HTTP syncing feature
which doesn't work with libevhtp 1.2.10 at all.
seafile: fetch sources using commit hashes instead of release tags
Avoids source filename conflicts and the need to delete source packages
at the end of the prepare stage. This has forced the buildenv to rebuild
the packages even if they were built already.
Signed-off-by: Gergely Kiss <mail.gery@gmail.com>