[compiler-rt] [compiler-rt] Don't redefine LLVM_COMMON_CMAKE_UTILS if it's defined (PR #66761)

[Fangrui Song via llvm-commits]

MaskRay wrote:

> If we are building compiler-rt inside llvm-x.y.z.src directory, we should not redefine LLVM_COMMON_CMAKE_UTILS.

LGTM. Can you add more instructions?

I can see that some subprojects checks `DEFINED` while libcxx/ doesn't check whether `LLVM_COMMON_CMAKE_UTILS` is defined

https://github.com/llvm/llvm-project/pull/66761