When building with ASan, core is also built with ASan enabled (and thus
using a local build is required). The other two sanitizers aren't also
forwarded on to core's settings since they don't need to be.
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.
Thomas Goyne
Mark Rowe