Detect the compiler vendor and disable -index-store-path if not Apple

[jakepetroules]

The clang -index-store-path is only in Apple clang, and not upstream LLVM. We need to detect whether we have an Apple clang, and only pass this flag to the compiler when that is the case.

This can be an issue on Linux, especially in Docker containers, where Apple's clang may be installed at /usr/bin/clang and then subsequently overwritten by the system package manager which also owns that same path for one of the packages supported by the distro.