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.

It's not a dependency. If the directory doesn't exist, things continue to work exactly as they do now.

> Moreover, ../tightdb assumes that everybody has the same directory structure.

The entire point of this is to remove the need for configuration by making things work when they're set up in the normal way. If someone insists on some other directory structure for some reason, they can continue to do whatever it is they're doing now.

> As @kspangsege mentions, build.sh already supports using a locally compiled core.

As I said in the PR's description, there's no good way to use that support from Xcode.