JSC should build cleanly on its own (e.g. no dependencies on anything on top of it.) It's possible some of the build scripts have developed a bizarro dependency on other portions of the repo, but we'd happily take patches to fix those issues.

As far as APIs, JSC has provided a completely API and ABI stable C API for 7-ish years, and there's an ObjC API built on top of that now. Unfortunately the nature of C APIs for interacting with language runtimes makes it somewhat clunky and hard to use perfectly - C++ makes this easier as you can make types with constructors and destructors to make a lot of the otherwise manual work (ref()/deref(), etc) entirely automatic. Unfortunately C++ makes it easy to make an API that has a stable interface, but still isn't ABI stable.