check that policy CMP0087 exists in python/CMakeLists.txt file and set it to new if available

python/CMakeLists.txt

@@ -5,6 +5,6 @@ dune_python_install_package(
   CMAKE_METADATA_FILE dune/femdg/metadata.cmake
   CMAKE_METADATA_FLAGS DUNE_OPTS_FILE
 )
-if ("${CMAKE_VERSION}" VERSION_GREATER 3.14)
+if(POLICY CMP0087)
   cmake_policy(SET CMP0087 NEW)
-endif ("${CMAKE_VERSION}" VERSION_GREATER 3.14)
+endif()