[CMake] Set the CMP0114 policy

[phosek]

Summary

We may consider enabling this policy in the future, but for now just
suppress the warnings to preserve the backwards compatibility.

[smeenai]

Makes sense to stick to the old behavior (which we know to work) for now. The new behavior seemed desirable for the future, though I haven't fully understood it yet.

[ldionne]

I'm using cmake version 3.20.3 and I don't think I've seen any warning so far. What happens on your side that prompted you to make this change?

[phosek]

In D111748#3063987, @ldionne wrote:

I'm using cmake version 3.20.3 and I don't think I've seen any warning so far. What happens on your side that prompted you to make this change?

We see a lot of the following warning:

CMake Warning (dev) at /b/s/w/ir/x/w/cipd/share/cmake-3.19/Modules/ExternalProject.cmake:2240 (message):
  Policy CMP0114 is not set: ExternalProject step targets fully adopt their
  steps.  Run "cmake --help-policy CMP0114" for policy details.  Use the
  cmake_policy command to set the policy and suppress this warning.

  ExternalProject target 'stage2' would depend on the targets for step(s)
  'really-install' under policy CMP0114, but this is being left out for
  compatibility since the policy is not set.
Call Stack (most recent call first):
  /b/s/w/ir/x/w/llvm-project/clang/CMakeLists.txt:858 (ExternalProject_Add_StepTargets)
This warning is for project developers.  Use -Wno-dev to suppress it.

Here's an example: https://logs.chromium.org/logs/fuchsia/buildbucket/cr-buildbucket/8833340663294366177/+/u/clang/configure/stdout

[ldionne]

Hmm, alright. Haven't seen it yet but I don't object.

[ldionne]

[Github PR transition cleanup]

I think this has been done now, I can see both being set in cmake/Modules/CMakePolicy.cmake.

llvm/CMakeLists.txt

 # See docs/CMake.html for instructions about how to build LLVM with CMake.
 
 cmake_minimum_required(VERSION 3.13.4)
 
+# CMP0114: ExternalProject step targets fully adopt their steps.
+# New in CMake 3.19. https://cmake.org/cmake/help/latest/policy/CMP0114.html
+if(POLICY CMP0114)
+  cmake_policy(SET CMP0114 OLD)
+endif()
+
 # CMP0116: Ninja generators transform `DEPFILE`s from `add_custom_command()`
 # New in CMake 3.20. https://cmake.org/cmake/help/latest/policy/CMP0116.html
 if(POLICY CMP0116)
   cmake_policy(SET CMP0116 OLD)
 endif()
 
 set(CMAKE_BUILD_WITH_INSTALL_NAME_DIR ON)