The dynamic library makes it easy to verify that there are no linker
errors when building the object store, while the static library is
easier for a binding to consume.
This also tweaks how the library targets are defined to ensure that
other CMake projects that pull in the libraries automatically get the
right include paths and link to the appropriate libraries.
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