[libclc] [llvm] [libclc] Refactor build system to allow in-tree builds (PR #87622)

[Fraser Cormack via cfe-commits]

================
@@ -27,31 +36,51 @@ set( LIBCLC_TARGETS_TO_BUILD "all"
 
 option( ENABLE_RUNTIME_SUBNORMAL "Enable runtime linking of subnormal support." OFF )
 
-find_package(LLVM REQUIRED HINTS "${LLVM_CMAKE_DIR}")
-include(AddLLVM)
+if(CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR)
----------------
frasercrmck wrote:

Yeah, I think you're right. I'm not 100%, but other LLVM sub-projects do it so I think we should too.

https://github.com/llvm/llvm-project/pull/87622