How should one set cmake ar/ranlib

Question

I'm using the following variables. It seems cmake do use clang but ar/ranlib is not properly set.

//cmake version 3.18.2

SET(CMAKE_AR           ${tools}/bin/llvm-ar)
SET(CMAKE_RANLIB       ${tools}/bin/llvm-ranlib)
SET(CMAKE_C_COMPILER   ${tools}/bin/clang)
SET(CMAKE_CXX_COMPILER ${tools}/bin/clang++)

My attempt at an reprex:

checkout llvm-project 7fa503ef

$ mkdir clang1
# build clang here

$ cd <...>/llvm-project
$ cat ./my-toolchain.cmake

SET(CMAKE_SYSTEM_NAME Linux)
set(tools ./clang1)
SET(CMAKE_AR           ${tools}/bin/llvm-ar)
SET(CMAKE_RANLIB       ${tools}/bin/llvm-ranlib)
SET(CMAKE_C_COMPILER   ${tools}/bin/clang)
SET(CMAKE_CXX_COMPILER ${tools}/bin/clang++)


$ mkdir clang2; cd clang2
$ cmake -DCMAKE_TOOLCHAIN_FILE=../my-toolchain.cmake -DLLVM_TARGETS_TO_BUILD="X86" -DLLVM_ENABLE_PROJECTS=clang -G "Unix Makefiles" ../llvm
$ cat ./lib/Demangle/CMakeFiles/LLVMDemangle.dir/link.txt

Tried this from @Tsyvarev

SET(CMAKE_AR           ${tools}/bin/llvm-ar CACHE FILEPATH "ar")
SET(CMAKE_RANLIB       ${tools}/bin/llvm-ranlib CACHE FILEPATH "ranlib")

https://cmake.org/cmake/help/v3.18/command/set.html

Still getting -- Wrong!

/usr/bin/ar
/usr/bin/ranlib

I'm sorry. The trick did work!