Avoid installing external LLVM dylibs

Avoid installing external LLVM dylibs

If the LLVM was externally provided, then we don't currently copy artifacts into
the sysroot. This is not necessarily the right choice (in particular, it will
require the LLVM dylib to be in the linker's load path at runtime), but the
common use case for external LLVMs is distribution provided LLVMs, and in that
case they're usually in the standard search path (e.g., /usr/lib) and copying
them here is going to cause problems as we may end up with the wrong files and
isn't what distributions want.

This behavior may be revisited in the future though.