[CMake] Explicitly set CMP0116 policy

[Kayjukh]

Summary

CMake 3.20.0 introduces a new policy that allows us to specify the
behavior of the DEPFILE argument to add_custom_command() for Ninja
builds. Setting this policy to OLD makes DEPFILE be either an absolute
path or a path relative to CMAKE_BINARY_DIR. In contrast, the NEW
behavior makes the path absolute or relative to CMAKE_CURRENT_BINARY_DIR.

CMake only emits a warning because DEPFILE is used in a subdirectory. If
it were not, the build would default to the OLD behavior. Explicitly set
the policy to OLD to silence this warning for Ninja builds.

Additional information: cmake --help-policy CMP0116

[Kayjukh]

Check for the CMake version before setting the policy. Unknown policies cause errors during the CMake configuration phase.

[stephenneuendorffer]

Seems reasonable, but maybe it would be worthwhile documenting in the code why it is being set?

[Kayjukh]

This revision slipped under my radar and the new policy change was addressed by https://reviews.llvm.org/D101083. Closing.

llvm/CMakeLists.txt

 # See docs/CMake.html for instructions about how to build LLVM with CMake.
 
 cmake_minimum_required(VERSION 3.13.4)
 
 set(CMAKE_BUILD_WITH_INSTALL_NAME_DIR ON)
 
+if(${CMAKE_VERSION} VERSION_GREATER_EQUAL "3.20.0")
+  cmake_policy(SET CMP0116 OLD)
+endif()
+
 if(NOT DEFINED LLVM_VERSION_MAJOR)
   set(LLVM_VERSION_MAJOR 13)
 endif()
 if(NOT DEFINED LLVM_VERSION_MINOR)
   set(LLVM_VERSION_MINOR 0)
 endif()
 if(NOT DEFINED LLVM_VERSION_PATCH)
   set(LLVM_VERSION_PATCH 0)