Allow for optionally unbundled leveldb from CMake builds.

Following the example of unbundled libevent and libarchive, we should allow for unbundled leveldb if the user wishes so.

For leveldb, this task is not as trivial as one would hope due to the fact that we link leveldb statically. This forces us to satisfy leveldb's strong dependencies against gpertools (tcmalloc) as well as snappy.
Alternatively, we would resort into linking leveldb dynamically, solving these issues.