[BUILD] require cmake 3.18, make policy conditional (#1303)

According to
https://cmake.org/cmake/help/v3.0/manual/cmake-policies.7.html, just
setting policy shouldn't require changing minimum required CMake version
(moreover, setting minimum cmake version by default sets all new
policies to True)
cc @malfet

CMakeLists.txt

@@ -1,8 +1,10 @@
-cmake_minimum_required(VERSION 3.20)
+cmake_minimum_required(VERSION 3.18)
 
+if(POLICY CMP0116)
 # Introduced in cmake 3.20
 # https://cmake.org/cmake/help/latest/policy/CMP0116.html
-cmake_policy(SET CMP0116 OLD)
+  cmake_policy(SET CMP0116 OLD)
+endif()
 
 include(ExternalProject)