Don't download the core when a local build is present

There's no particularly pleasant way to set REALM_CORE_VERSION when
building from Xcode, so try to automatically detect that the copy of the
core was built locally and leave it alone if so.

@alazier @emanuelez @jpsim 
I don't think we should add an dependency (`../tightdb`) to a private repository. Moreover, `../tightdb` assumes that everybody has the same directory structure. As @kspangsege mentions, `build.sh` already supports using a locally compiled core.