Use WebKit's copy of leveldb when system version is not available

To support indexed database webkit needs leveldb, therefore webkit
contains a copy of leveldb which we can use to provide support when
the system doesn't ship with a suitable version.

Change-Id: I231694e6a7c7cb83855bfed3f9ee24fbd2795362
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>