Configuring with cmake 3.12 emits the following warning and ignores LLVM_ROOT.
Fix this by explicitly requiring the new policy.
CMake Warning (dev) at cmake/FindClang.cmake:46 (find_package): Policy CMP0074 is not set: find_package uses <PackageName>_ROOT variables. Run "cmake --help-policy CMP0074" for policy details. Use the cmake_policy command to set the policy and suppress this warning. CMake variable LLVM_ROOT is set to: /d2/llvm/9/vanilla/installs/Release For compatibility, CMake is ignoring the variable. Call Stack (most recent call first): CMakeLists.txt:26 (find_package) This warning is for project developers. Use -Wno-dev to suppress it.