By default, Linux uses a locally-built version of core that is prepared
in an independent working copy of the realm-core git repository.
Support is also added for using an existing local build of core on both
OS X and Linux . This can be done by running
`cmake -DREALM_CORE_VERSION=/path/to/realm-core`. The generated build
system will invoke `sh build.sh build` within the given directory prior
to building the object store.
Mark Rowe
Thomas Goyne