diff --git a/llvm/CMakeLists.txt b/llvm/CMakeLists.txt
--- a/llvm/CMakeLists.txt
+++ b/llvm/CMakeLists.txt
@@ -2,6 +2,12 @@
 
 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)
diff --git a/runtimes/CMakeLists.txt b/runtimes/CMakeLists.txt
--- a/runtimes/CMakeLists.txt
+++ b/runtimes/CMakeLists.txt
@@ -2,6 +2,18 @@
 cmake_minimum_required(VERSION 3.13.4)
 project(Runtimes C CXX ASM)
 
+# 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(LLVM_ALL_RUNTIMES "compiler-rt;libc;libcxx;libcxxabi;libunwind;openmp")
 set(LLVM_ENABLE_RUNTIMES "" CACHE STRING
   "Semicolon-separated list of runtimes to build (${LLVM_ALL_RUNTIMES}), or \"all\".")