Make lcov/gcovr an optional dependency that's only needed for Coverage
configurations, remove some pointless noisy messages when not generating
coverage, and generally simplify the whole thing.
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
Thomas Goyne