There's quite a lot of references to Polly in the LLVM CMake codebase. However the registration pattern used by Polly could be useful to other external projects: thanks to that mechanism it would be possible to develop LLVM extension without touching the LLVM code base.
This an attempt at doing so, using Polly as reference.
Note that there is LLVM_POLLY_LINK_INTO_TOOLS and LINK_POLLY_INTO_TOOLS. The former is the user-configurable option, the latter is for internal querying. At least, this is what is was meant for.